The 52 Mantle is only uncommon in the very top grades. With just over 1300 graded by PSA alone I don't think I'd even call it uncommon. Its price is almost entirely because of demand.
I have a few altered cards. Some I knew were altered when I bought them, others I didn't. I don't even count the minor stuff that I don't think of as an alteration. Soaking scrapbook remnants off, light surface cleaning(NOT stain removal, just removing years of filth that's on the surface) Removing wrapper wax from the front, that sort of thing.
I do think there are alterations that get past TPG. With the setup they all have it's just not possible to catch the really well done ones every time. If they took more time they'd catch more.
At the same time undersize isn't always trimmed. But beyond a certain point an undersize card shouldn't be slabbed under the current system. If they had a qualifier for "undersize but not trimmed" it might be doable.
I'd like to see SGC use the rejection code flips instead of the "A". I think 3 of the 4 I've had rejected were unaltered but either undersize or had cuts too rough to grade. The trimmed one was trimmed for sure - I still can't figure out why I sent it in.
In most collecting original is always worth more than restored. Even cars, and a lot of the bicycle guys I know have come around from the older "restore everything" mentality. Original with a couple scratches is much better than restored.
Btw - there apparently are or were multiples of some famous art. They mentioned it on a show I saw about a possible second Mona Lisa. The artists studio would sometimes make smaller copies of larger works that they could sell a real profit center for some artists.
There's a lot of cards I'd be happy to have even altered. It's just a matter of balancing the price for altered compared to the price of an unaltered copy. And that of course requires disclosure.
A full price restoration? I'd have to think on that one. Probably not for me, I've never been too condition crazy.
Steve B