Fraser -
You are correct, the INS's had a access to a separate memory facility in later years of operation. The access method is as you described, and then a numerical entry to access the relevant part of the route (or Flight Plan Segment), which would be numbered between 01 and 87.
The comms log would list the appropriate FPS number. Obviously, it was still possible to enter all waypoints longhand.
It was also possible to access a DME lat/long database using the same system which enabled DME updating and saved a lot of finding/typing Lat/Long for DMEs. (These were numbered between 90 and 99).
FWIW there was a specific Delco variant for supersonic flight (Delco IV-AC). It was permissible to have a 'standard' IV or IV-A in the number 3 position. IIRC the aircraft would be restricted to subsonic flight unless there was a IV-AC in both 1 and 2 positions.
Before my time, I believe there was a card reader system to do a similar job.
Stilton -
It did have a well-disguised FMA..... the Mode Select Panel was also the FMA. Lit button = active mode.