Note from a little farther down on the Serial Number scheme - confirms what we already knew:

Notes:

1. The 531DB Gran Sports built in the early 70's did not have letters in the serial number.
2. Frames built at the Ilkeston facility have serial numbers that begin with "SB".
3. The 1973 track frames begin with an "A".
4. There are several documented cases of '73 and '74 Professionals, Internationals, Competitions and RRAs with "A" serial numbers. Let the color scheme, decals and components be your guide.

I have a '73 Competition frame with a fork with problematic threading. Need to figure out a solution for it, as I'd really like to build and ride it.