A water transfer of that age would probably be extremely fragile, so I doubt it could be slid off without breaking up.

Equally, you can't scan it as is either!

Often, transfers were not so much for the actual bikes, but maybe for a car windscreen and just showing support for the bike?

I have a box of stickers and transfers, but stickers as we know them are relatively modern. My 1966 BRM H16 transfer is just that, a transfer. The Marlboro-BRM set were stickers, as were JPS Lotus, so I'm guessing that just about anything pre 1970 is a transfer - unless of course you have been a sold a repro item that doesn't actually date back to the 1920's anyway.