what engine does my bantam have?

Hello all, im not sure what engine my bantam has. It has been trial-ised so it has no obviouse decals. The engine number is BD3B 25037 Is it a d1, d2, d3?? It has plunger rear suspension.

Many thanks, Jim>

Engine "BD3B 25035" appears to make it a D3 from the 1957 series (BD3B 18698+) http://bsaownersclub.co.uk/Engine_Frame_Listing.html