Lippard's book is a good start but there is some mis-information in there but not any great consequence. There are "standard" models but Perazzi would (and may still) provide the client with any combination of features they might desire. The important piece to know is that parts are available for anything but the ejectors for the very earliest Type 1 guns (AFAIK those ejectors are impossible to find even from Perazzi). There are four basic action types but the commonest is the MX8 type with the dropout trigger. And the internals are all the same with the exception of the hammer/spring parts for the coil spring guns. Changing from top first firing to bottom first firing requires only one small part but it MUST be fitted by someone who actually knows what they're doing. Older guns may require new parts to be fitted, but it has been my experience so few replacement parts are ever required that it becomes a non-issue and most parts will just drop in.
The ones in the accumulation here run from 1972 to 1984. Most have tens of thousands of carts thru them and all are alive and well.
The top first thing was an artifact of the "top barrel recoils more" thinking, something which is just BS AFAIC. But it does in some instances make guns less desirable and that has been financially advantageous to me several times. A big thanks shout out to all the meatheads that have been so generous to me in that regard. Anyway, since the top barrel recoils more (hahaha) you put the lighter pigeon load in it and the cannon load second barrel in the bottom that recoils less hahaha. When bunker shooters were still using 36gm and 32gm loads the thinking was prolly the same hence the MX2. You could have any gun provided new as a top first and I have a Mirage that is just that and even has a factory tube in the bottom barrel to give the gun some real versatility. Obviously the original owner was a pigeon shooter. The MX3, MX1, and MX2 were all top first guns in the old catalog. The MX1 that I have is early '80s and was certainly a pigeon gun order. The older MX1 and MX2 both had the same height high rib and I've had a MX2 bunker barrel fitted to the MX1 and me and that gun are just great friends.
This is not to sound pompous so please ............... anyway I've been gathering info on the guns as simple self protection for a couple decades so if anyone has a specific question I may be able to provide some info. Or point you to someone who can.
Charlie