Maybe it was because they were manufactured / renovated in different places (factories)? I don't remember exactly, but try to find a National Geographic documentary from the Megafactories series about Abrams renovations. The renovation is taking place in two factories (Anniston and Lima, as long as I haven't screwed anything up).
Update: Here you can find some photos ( they looks like these on the NG document ):
https://www.businessinsider.com/rebuild ... 13-11?IR=T
It could happen that the lower hull damaged by the IED was not fit for repair and was written off, while the turret remained undamaged / functional (or vice versa). The Abrams, as only a few US combat vehicles, are so valuable that they return to the US for repairs after such an accident.
In addition, it is mainly the tower that receives the most modernization, and the old M1 tanks stored and preserved by the Army are used to build "new" tanks (including those for export). They are undergoing a major overhaul (in fact, apart from the pure hull, they get everything new) and they get a modernized turret (also equipped with all new components ) with new special armor inserts.
P.s. Depending on the special armor used, a letter is added after the serial number on the turret. For depleted uranium, it is the letter "U". But from what I remember, the modern version of SEP V3 no longer uses depleted uranium, but a modern equivalent of composite armor (hence the elongation of the turret armor and the front of the lower hull) ... Unfortunately, I don't remember whether the new special armor has any letter markings ...
P.s.2. Very nice numbers came out for you, but in reality they are not even half as beautiful as yours

They are simply made by a welder with a welding wire - never twice the same
Good job btw
