Originally Posted by
JohnDThompson
If your frame is Reynolds 531, and has the Shimano UF semi-vertical dropouts and Cinelli bottom bracket shell, it is an early production 750 frame. Those dropouts and shells were only used on the 750/950 series frames.
I'm quite confident it is a 750 but the serial number date of 1978 and the A prefix don't fit in with the normal serial number pattern. Is it possible a 750 was made in 1978? My understanding is that the 750 was designed by Tim Isaak and he wasn't hired until 1979 according to the Trek timeline.