 The early commercial SP-1 AR-15s used a pair of .250" diameter receiver push pins, identical to those found on the military rifles.In 1966 the company replaced the front pin with a paired nut and screw hinge using a .315" diameter pin to prevent shooters from being able to change receivers with military rifles or competitor rifles without the use of an adapter.They resumed production with the smaller and standardized .250" pin in the mid-1990s.