Rob you're not a dumbass.
They're not m101 though, so your reference to that set is misplaced-- the cards are akin to E121, Holsum, D350-3 Standard Biscuit, Shotwell, Queen City, w575 etc.
I'll let Rhett and others who focus on these sets elaborate but will give my two cents. As I understand your question, you wonder whether legitimate card fronts from 1921 had their backs added later. This has always been a concern of mine with stamped backs, as anyone can have a stamp cheaply created to say most anything, and then the only trick would be getting the right ink. As mentioned, some known Henry Johnson and a couple of advertised chocolate brands using these card fronts have surfaced which are known to be fake in this regard.
These Herps were printed--all are centered and there is no uneven inking. To print them as you ask would require that you first have an uncut sheet of genuine blank-backed e121s, a rare enough piece in its own right. Why anyone would defile such an item with fake backs and then cut it into cards to create fantasy pieces defies logic. Moreover, and I'll let those with printing experience take over here, it would be very cost ineffective and expensive to set up the printing process for such an endeavor, particularly to produce just one sheet. This makes even less sense if done 40 years ago with no likelihood or even creator's desire of recouping money. Finally as I understand it, there are some of these Herpolsheimer's that depict players who are unknown to e121, so it's even less possible someone could have even used a sheet of blank-backed e121s, leaving perhaps only w575s as candidates, which themselves may have been strips, not sheets. Of course, I have not even touched upon the expertise required to adequately replicate the design, ink and fonts to look like those used in some of the other 1921 sets.
I think you also have to ask yourself, if someone really had used a legit sheet of blank-backed cards and just added very realistic period-looking backs, how in the heck would a dealer be able to so easily dismiss them as fakes? What analysis was undertaken–if the fronts are real then the “tell” must be on the backs, right? What is it? What did they know 40, 20 or 15 years ago that we don’t know now?
No, I stick to my theory that someone just dismissed them because they were unknown/ uncatalogued and because other known fakes using some of the same fronts (but different backs) had been spotted and were on the hobby radar at the time.