I haven’t been able to find any images on it either, just the XM291-armed variants (120mm/140mm) and a powerpoint image.
Edit: If no images can be found of the vehicle, it may be easier to convert the suggestion the M1A2 140mm variant (the images in the suggestion) or the M1A1 120mm variant (seen below).
Some extra pictures from the program. I’m still not certain if the gun in the picture above Yontzee posted is the M256E1 or not, but there’s another picture of the same vehicle regardless.
Agreed, OP said that photo and the other one were in the same file for the M256E1 upgun program, and wasn’t sure of which gun it was, but I think the general consensus is that it’s the XM291.