OB HP = Optima HP Choke; OB or OBS = Optima Choke; OBF, OCS = Optima Plus Choke; MC or PB = Mobil Choke; MOBIL: 2” from gas skirt to top of thread, threads flush to muzzle. 302, 303, 304, gold, 390, 391 URIKA (not Urika 2…Urika 2 takes Optima), A-300, 687, Silver Pigeon 2 and 3 (before 2011), SOME 686, Onyx Sporting, 682, 682 Gold (682 Gold E takes Optima), Most of their field/hunting ...