Here are some details that I found in a magazine article about the F.27/F27.
The Fokker F.27 was built in the Netherlands, and the Fairchild F-27 was built in Hagerstown MD. Fokker and Fairchild agreed in 1956 on a joint production plan. First flight of the prototype F.27 was in Nov '55...the first production standard F.27 flew in Jan '57. The Fairchild F-27 first flew in April '58, with Dick Henson as the test pilot. The F-27 was type-certified in July of '58 and entered service with West Coast Airlines in Sept. The F-27 actually carried revenue passengers before the original F.27.
Fokker made further developments to the design... the F.27 Mk 100 through the final Mk 500 had various versions of the RR Dart, fuselage lengths, weights, doors, etc. The last F.27 built, a Mk 500, was delivered to Air Wisconsin in 1986. A total of 581 were built.
Fairchild made similar developments with the F-27A, B, F, J, and M. In 1966, Fairchild/Hiller first flew the FH-227, with a 6' fuselage stretch over the F-27. The FH-227 had several versions up through the D model. Fairchild built a total of 126 F-27s and 79 FH-227s when production ended in 1973.
Though the airplanes looked similar, they were actually very different. Most airlines flew either one type or the other, because each required different manuals and spare parts.
Some visual differences:
F.27s need boarding stairs, while F-27s have integral steps in the doors.
F.27s had pitot/static tubes on the wingtips, F-27s were on the nose.
F.27s had heat exchanger intakes on the lower aft fuselages, while the F-27 intakes were smaller and mounter up on the fin (stretched models only)
Counting windows is difficult because of the various versions and fuselage lengths.
An F.27 airframe was used to build the F-50 prototype, first flown in 1985. As Metrodriver mentions, it had PW125B/127B engines in new nacelles, six-blade props, EFIS cockpit, smaller windows, twin-wheel nose gear, forward pax door, 'Foklets' (small winglets), and completely new electric, air-conditioning and hydraulic systems. Production ended in 1997.
It's official....I have waaaaaay too much spare time tonight.