BSA Model's Identification

I would like to help a friend for finding a piston in order to fix his BSA.
I am not sure of the model so I took some pictures.
Which model could it be?
Where could I find a piston for its motor?
I noticed 2 numbers
On the fork: BC 11S4-14760
On the motor's block: BC 11 G, 20150
Thanks for your help.