All the drawings I have the circled numbers that start ‘A’ – ‘G’ are cross referenced with specific drawing numbers which invariably relate directly to part numbers, as others have stated. The letter refers to the specific size/scale of the drawings.

As an example I recently came across a photograph of a Hawker Typhoon part, it was believed to be a from the wing. The part number D96131/3 was stamped on it. I have this drawing number (the slash representing port/starboard) and I could identify this as part of the rear spar for the tail plane.