I've been collecting Topps print anomalies for a long time and nothing looks like that one that I've seen.
You can rule out a wet sheet transfer if both images face the right way. A wet sheet transfer is always backwards because it is just a stamp of the correctly printed card surface on top of the card that picks up the transfer:
It is also always a single color, because colors were applied singly, and the odds of all of the inks being wet are non-existent.
The cards are printed using transfer technology: the ink on the plate transfers to a rubber mat and the mat presses onto the cardboard, which makes the image face the right way on the cardboard. It is like looking at a mirror image in a mirror: ><> On rare occasion, the plate prints directly onto the stock instead of the rubber mat (no transfer of ink) and you get a vivid backwards print of one color:
In that one, you can see that the Killebrew front is missing one of the black ink passes and that it got the back blacking on the front instead, but in reverse. The sheet went through the press wrong-side up and got between the plate and the mat. Never should have gotten out the door.
It is possible to get a situation where the company mistakenly reprints one sheet over another and no one catches it. That usually results from a sheet being put into another stack of sheets after having been printed already. Here is a Bowman:
You can also get a 'double-struck' sheet where the black is run twice by accident. That results in an offset doubled set of black lines. I have a few but no scans to share.
Now, if print issues were all the issue was with the Mantle-Killebrew, I'd say the card may be OK. But being from two separate series and it happens to be two of the biggest stars? Something is fishy there. What I suspect is that someone took a Killebrew and printed a Mantle image on its back.