what is it

[chris davies]

hi i have bought bike in bits can you help i.d it for me       eng'no  da10 16161  frame no'  ga7.17900  not sure of reg no' can you help  many thank's chris

[L.A.B.]

Both the engine and frame numbers seem to show that it is most likely to be a 1961 650cc BSA A10 Golden Flash (although some call them 'Gold' Flash which is not really correct).
http://www.bsaoc.demon.co.uk/post1960.html
There are some BSA A10 photos in the pictures section.
http://www.vintagebike.co.uk/Bike%20Directories/BSA%20Bikes/BSA%20index.htm
If it has never been registered on a V5 (now V5C) registration document then you may not have any claim to the original registration number even if you knew it? You may have to apply to the DVLA for an 'age related' number.
As you have the frame number you could possibly ask the DVLA to check their records although it is unlikely that they will find anything unless it has been registered on a V5?
http://www.dvla.gov.uk/vehicles/vehicles.htm
Are there any old tax discs with the bike as these should show the original registration number?