The “Heavy” armor piece from FMG was originally listed as the Semovente 75/18, which I believe is what it is.
At some point, someone put a post saying “the gun barrel was such-n-such long” and “the body was so-n-so wide” so it MUST be a 105/25. I imagine that is where HBG’s confusion lies. Coach must have seen that post and it stuck in his head so he put it on his site as such.