There Is A Phase I And Phase II...

...that much I know. Manufacturing date range is 1972-1977.

I recently scored two 454s, one of each Phase. The Phase II has lighter colored grips and a little ramp ground into the frame to make the BBs a bit easier to load. The Phase I has a screw head/coin slot to tighten the CO2, while the Phase II has on of those thumb tabs. My assumption is that the Phase II is a later model.