Interestingly, the S/N issue with the Garand didn't used to be illegal, depending on how the ATF determined the reason for the defacement and such.
I researched this very issue a few years back, and the ATF's position, as read from several AFT letters on the subject, changed over time.
At one point, for example, if the defacement wasn't performed by you (as in, say, a stolen gun that was recovered and returned to you), then this was allowed based on the level of defacement, and so forth. And the gun was allowed to be re-serialized...either by restoring the serial number or assigning a new, unique serial number.
Then, as I progressed through my research, it seems to me that I recall another ATF ruling in which it flat out wasn't acceptable at all, including re-serializing the firearm. And you simply couldn't legally own it.
Here is a link to one ATF ruling:
http://s940.photobucket.com/user/Gunny_2009_album/library/BATFE_rules?sort=3&page=1
In this ruling, the ATF has essentially ruled that if there was no finding that the owner was not at fault, the firearm could be restamped with a new serial number issued by the ATF. And I believe a record of the applicable documentation is supposed to be retained.
But in the end, I think your assessment of thus guy's "number 25" is probably correct, given what all you observed. I certainly wouldn't tough it with a ten foot pole.