The only answer I can really give at the moment is, It’s Complicated.
One issue being, there were multiple prototype versions of the round. Leading to part of the problem being the initial implementation of the round in game, was partially based on the penetration values for the first prototype version of the round at 60°. A great deal of that had to do with lack of available reliable sources at the time.
Table of Information I've been able to find
| Name | Jacket/Body | Core | Density | Core Mass | Core Length | Fin Assembly | Ballistic Cap | Mass Flight | Length Flight | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| XM735 | Steel(Maraging) | Kennametal W-2 WHA | 18,500kg/m³ | 4.35lb(1.973kg) | 8.8in(223.5mm) | Steel(Maraging) 6-fin | Aluminum(Steel Tip) | 8.00lb(3.63kg) | 17.8in(452.12mm) | Adaptation of XM579E4 Projectile body to the 105mm M68 Gun, revised fin assembly, saddle type sabot, Initial version used at first Tripartite Gun Trials in 1974. |
| XM735E1 | Steel(Maraging) | Kennametal W-2 WHA | 18,500kg/m³ | 4.85lb(2.2kg)??? | 12.185in(309.5mm) | Steel 6-fin | Aluminum(Steel Tip) | 8.14lb(3.69kg) | ??? | Improved model, longer Kennametal W-2 WHA core (mass is questionable), and body |
| XM735E2 | Steel(Maraging) | Teledyne X11 WHA | 18,500kg/m³ | 4.85lb(2.2kg) | 12.185in(309.5mm) | Aluminum, 6-fin | Aluminum(Steel Tip) | 8.20lb(3.72kg) | 19.06in(484.12mm) | last major revision, aluminum fin assembly, improved ballistic cap, changed to Teledyne Firth Sterling X11 WHA for core with better all-around mechanical properties |
| M735 | Steel(Maraging) | Teledyne X11 WHA | 18,500kg/m³ | 4.85lb(2.2kg) | 12.185in(309.5mm) | Aluminum, 6-fin | Aluminum(Steel Tip) | 8.20lb(3.72kg) | 19.06in(484.12mm) | Standardized in 1976, Production revision Model of XM735E2 |
| M735A1 | Steel(Maraging) | NMI DU Staballoy(early) | 18,500kg/m³ | 4.85lb(2.2kg) | 12.185in(309.5mm) | Aluminum, 6-fin | Aluminum(Steel Tip) | 8.20lb(3.72kg) | 19.06in(484.12mm) | Initially standardized and ordered as a stopgap measure until M774 could enter production. Replaced WHA core with a DU core of the same mass and dimensions, cores were initially produced by Nuclear Metals Inc, Order Cancelled when XM774 Development and Testing were accelerated |
Needless to say, there were major improvements in performance, between XM735, and XM735E2 or M735. Such that XM735 could only perforate NATO Single heavy at ~1000m, whereas XM735E2 could do so at ~3400m against the Tripartite test plate.