There is some useful content on RUAG UK's web site
http://www.ruag.co.uk/downloads/41-the-perazzi-experience
About half down the page is a PDF detailing the difference between and MX8 and MX12 as well as another document on ageing a Perazzi. As others have noted they are beautifully engineered bits of kit and designed, from the outset, to be maintained for virtually ever.
There are a couple of facts that are useful to understand as you delve into the wonders of Perazzi that will save you a lot of head scratching in the future:
1) There are in essence two distinct families of Perazzi shotguns, namely those with a barrel selector and those without. If your shooting history includes just about ever other makers OU shotgun i.e. Browning, Miroku, Beretta etc then you would assume barrel selectors are the norm..... So MX8 typically don't have a selector (though I believe it was an option with a switch like effort behind the trigger guard) and MX12 do.
Lots of detail differences, different actions, MX8 usually have leaf springs for the triggers and MX12 has conventional coils - I've tried both, John Jeffries of HPX Perazzi tunes the triggers on his guns and I can't tell the difference but maybe I'm just insensitive :frown:
2) Whilst it's possible to buy a standard Perazzi off the shelf many are custom made to order (it doesn't cost any more), with a myriad of options i.e. there are 5 sizes of bore size for a 12 guage I believe, the tighter sizes are favoured for clay shooters and the looser for the game orientated guns. Ribs, heights, widths, tapers etc etc. Having had a couple of HPX's made to order I can say that it's impossible to tell from the documentation (you get a little book with the dimensions) that the gun was made for someone to spec.
As Wonko has indicated Karl Lippard's book is good but not perfect, it was after all Karl's own notes assembled over a number of years as a senior exec at Perazzi Inc, just published into book format if I recall correctly it ends in the mid 70s, still a fascinating insight into the evolution of the guns.