...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.
It does have the BB loading ramp. My CO2 piercing screw is a cylindrical headed bolt with a split keyring through it, probably someone's previous repair of a broken thumb tab. Works fine.
I replaced the seals in this gun a few years ago, with parts that were still available back then from Crosman. I don't remember much about the process, except that it took me several tries to get it back together and functioning. Kinda like working on a Cr 357; lots of parts that go boing and disappear across the room.
One thing I DID learn: if the gun suffers from BB mis-feeds and hang-ups in the magazine, it is probably due to bumps/knocks on the top of the action, and/or overtightening of the mating screws. In either case, the mis-feed problem can be solved by very careful use of a flat-blade srewdriver. Just insert it in the magazine observation slot, and open it up ever so slightly.