John, PBB is correct. However, your friend needs to know the exact and complete model numbers to know what he is comparing.
For example, PBB says the MX-10 and MX2000/10 are the same except for engraving, etc. That is correct.
Now compare the two models you asked about. If they are the complete model numbers, the are different guns. The MX-10 is a top single with a 3-notch adjustable rib. It can be ordered as a combo. An MX-2000 is an unsingle (bottom single) with a 5-notch adjustable rib, and it cannot be made into a combo without some compromise.
So you have to be precise when asking what are the differences between Perazzi models.
Thanks folks, we both thought the two models were much the same, except for the engraving. He is looking at several MX10 combos, and obviously all have top barrel singles, which is what he wants. I thought there was a MX2000 combo with a top single that has the adjustable rib on it and the O/U barrels...I assume it's proper designation in the MX2000/10? If he were to choose the MX10 over this, what would he be getting/not getting vs the MX2000/10?
The answer to your last question (seems like the same as your original question too) is in the first reply by PBB.
Seems there're always some confusions about the MX-2000, Basically MX-2000 is an upgraded MX-8. That being said, we have to remember all other standard grade O/U model in current production are based on MX-8 with added features on the stock and rib.
However, MX-2000 has (only) two models/configurations unique to itself are the unsingle (5 notches) ATA gun, and MX-2000/3 (3 notches O/U). Those are not an upgrade from any other models. It's simply "MX-2000 unsingle" & MX-2000/3 models. All other MX-2000 are an upgraded version from existing models.