First, my use of 135.31 above was incorrect. The proper reference is 14 CFR 61.31...not part 135 as I stated.
Four conditions are given as alternates to obtaining the endorsement. In each case, the pilot must be able to document the specified requirement. With respect to obtaining this certification as part of a Part 135 program, the specific requirement is to obtain a pilot proficiency check in a flight simulator or flight training device that is representative of a pressurized aircraft; the proficiency check must be a PIC check, and it must be conducted by the FAA, or a designated company check airman.
Merely receiving instruction from a company instructor does not meet the requirements of 61.31(g)(3)(iv). A company instructor who is not a certificated flight instructor cannot provide the high altitude endorsement required by 61.31(g). If one takes a PIC proficiency check under Part 135 and documents it, no endorsement is required. If one intends to obtain an endorsement, it must be provided by a authorized instructor. A company instructor who is not a certificated flight instructor is not an authorized instructor, and may not provide that endorsement, as it is a privilege and requirement of Part 61, separate from the functions of an instructor acting under Part 135. Again, taking the instruction at the company doesn't meet the specific requirement; passing the PIC proficiency check under 135 does, when it's given by the FAA or a company check airman...in which case no endorsement is required.
If one does pass the PIC proficiency check under Part 135 in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft, then one doesn't need the endorsement, but does need to be able to document having accomplished the check. This may be done in several ways, but is best accomplished by having a copy of the Form 8410 showing satisfactory completion of the proficiency check. One may also elect to obtain a signature or statement from the check airman in the log attesting to having met this requirement, or most preferably, obtain the actual endorsement in one's log (as noted by WMUSIGPI).
I see that previously you were asking if a company instructor who is not a CFI may provide this endorsement, under 135, and the answer is no. I misunderstood your question, and may have mislead you based on your actual intent. I have copied the regulation excerpt, with the relevant pieces emphasized for clarity, below.
So a company instructor can give the endorsement to another company pilot, and that endorsement will be valid for part 91 flights. They can not however give the endorsement to a non company pilot because they are NOT A CFI.
Four conditions are given as alternates to obtaining the endorsement. In each case, the pilot must be able to document the specified requirement. With respect to obtaining this certification as part of a Part 135 program, the specific requirement is to obtain a pilot proficiency check in a flight simulator or flight training device that is representative of a pressurized aircraft; the proficiency check must be a PIC check, and it must be conducted by the FAA, or a designated company check airman.
Merely receiving instruction from a company instructor does not meet the requirements of 61.31(g)(3)(iv). A company instructor who is not a certificated flight instructor cannot provide the high altitude endorsement required by 61.31(g). If one takes a PIC proficiency check under Part 135 and documents it, no endorsement is required. If one intends to obtain an endorsement, it must be provided by a authorized instructor. A company instructor who is not a certificated flight instructor is not an authorized instructor, and may not provide that endorsement, as it is a privilege and requirement of Part 61, separate from the functions of an instructor acting under Part 135. Again, taking the instruction at the company doesn't meet the specific requirement; passing the PIC proficiency check under 135 does, when it's given by the FAA or a company check airman...in which case no endorsement is required.
If one does pass the PIC proficiency check under Part 135 in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft, then one doesn't need the endorsement, but does need to be able to document having accomplished the check. This may be done in several ways, but is best accomplished by having a copy of the Form 8410 showing satisfactory completion of the proficiency check. One may also elect to obtain a signature or statement from the check airman in the log attesting to having met this requirement, or most preferably, obtain the actual endorsement in one's log (as noted by WMUSIGPI).
I see that previously you were asking if a company instructor who is not a CFI may provide this endorsement, under 135, and the answer is no. I misunderstood your question, and may have mislead you based on your actual intent. I have copied the regulation excerpt, with the relevant pieces emphasized for clarity, below.
(3) The training and endorsement required by paragraphs (g)(1) and (g)(2) of this section are not required if that person can document satisfactory accomplishment of any of the following in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft:
(i) Serving as pilot in command before April 15, 1991;
(3) The training and endorsement required by paragraphs (g)(1) and (g)(2) of this section are not required if that person can document satisfactory accomplishment of any of the following in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft:
(ii) Completing a pilot proficiency check for a pilot certificate or rating before April 15, 1991;
(3) The training and endorsement required by paragraphs (g)(1) and (g)(2) of this section are not required if that person can document satisfactory accomplishment of any of the following in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft:
(iii) Completing an official pilot-in-command check conducted by the military services of the United States; or
(3) The training and endorsement required by paragraphs (g)(1) and (g)(2) of this section are not required if that person can document satisfactory accomplishment of any of the following in a pressurized aircraft, or in a flight simulator or flight training device that is representative of a pressurized aircraft:
(iv) Completing a pilot-in-command proficiency check under part 121, 125, or 135 of this chapter conducted by the Administrator or by an approved pilot check airman.