121.434(h)(4)(ii) gives the option of refresher training as outlined in the carrier's approved training program, or having a check pilot determine that the pilot has retained an adequate level of proficiency by observing that pilot in a supervised line operating flight. In essence, satisfactorily completing a line check. There are no provisions for extending beyond 150 days. A new proficiency check would then be required. Paragraph (5) lists the circumstances which a carrier my apply for authorization to deviate from the 100 hrs/120 day requirement of paragraph (g).