A very rare coincidence I would say.
The factory despatch books for the A50/A65 are recorded frame number first 101 up. The engine numbers are not issued in order.
The A65 engines with no extra letter suffix or prefix ended in about September 1963 somewhere around A65 4000, when the numbers adopted the form A65A, A65B, A65C etc.
The frame numbers from Sepember 1963 started at A50 5501 (according to Glass's Motorcycle check book 1958-1965 edition) but these would include A50 as well as A65 engines.
A65 engine/frame left the factory with the same numbers from around March 1966.
The only way to establish whether your bike engine and frame numbers match those it left the factory with would be by obtaining a dating certificate from the BSAOC Club Librarian.