First off the only guns you have to worry about are the early ones with the two larger pins at the back of the receiver --
When looking at the breechface, if a gun is unmodified it will be smooth. The "update" guns will show two hex key head screw heads at the bottom corners of the breechface. The early basic Field, Skeet, Trap and "Special" Trap guns that would need the factory update all just had the dog roll-stamping on the sides of the receiver. Once guns started coming from the factory with the update they added a wreath around the dog --
By 1977 the lower pin was gone from the sides of the receiver --