I believe that at some point there was a heavy paper sheet inserted between each large uncut sheet to facilitate cutting and/or keep the subsequently cut silks separate and "manageable". While I don't recall ever seeing examples in the S74-2 (or in the NS smaller examples) - I own some packaged/produced/distributed in such a manner and retaining the "backing" for 5/6 large silk series. I sold two such superior examples from the lg. BB series around 2000. Wonder if that's the backing you're referring to - I would believe any sheets issued contemporaneously to my examples whatever the individual size MIGHT be similarly produced. As a counter inference however - I have a complete "as issued" set of mint small indian silks and they were sent without such a "backing".
I have a complete set of the similar colored presidents (not a political statement obviously!) which are relatively plentiful and issued at the same time - studied them pretty closely in the past and no evidence of a backing. Similar to both smaller BB issues are a pair of actress sets that were offered in two series - one "wit" and one without (if you're from Jersey you can get away with that). The ones with backing are distinct and I've handled over 1000 of the colored ones - never any evidence of a backing. At one time hobby friend David Grimes (RIP) had a few hundred S74-2's and none were so backed. Hard enough to figure out what happened last week in my own house let alone over 100 years ago.
If by the following statement "I have seen and owned a few examples with blank cardboard backs" you mean they were actually GLUED (or show evidence of it) in the same manner as the S74-1's I would be surprised but still inclined to come up with an alternate reason
I have a fair amount of exposure and experience with silk issues although admittedly not very familiar with the vagaries of the baseball series. I have not seen or encountered any evidence of any other series aside from the S74-1 in any of the other small to medium silk issues having such a backing (exception to the actresses).
Certainly at this point in the ongoing foraging of the scrapbooks that detail past lives which might include these little swatches of ephemera - examples would have been unearthed with the backing - hard to believe otherwise.
Of course now I'm going to have to figure out which of the two sets were issued first - as a pure guess it would be the backed set came first and proved to be too expensive or convoluted to produce. Pure conjecture - the BB collector is likely saying to themselves Henry is an idiot because everyone knows the colored set was issued first. So be it.
If you or anyone else have any such example I'd love to see it so I can spend some time thinking it through again. I have no life so it would be a blessing.....
No guarantees whether written or implied...........