Do a Google on Stevens M 512, see if that returns any useful info? Appears to be a Turkish-made shotgun, probably will need to get the chokes from Savage/Stevens?
Did a little "look see" on interchange. Depending on the source, it may take a Beretta/Benelli standard threaded choke?
The older Charles Daly Turkish O/Us list interchange with Mossberg/Winchester/Browning threads.