T206 back marking pair

[t206hound]

Just picked up this Alperman on Ebay:


Here is a photo of the Reulbach (glove showing) I already own:


I think that it is highly probable that these two cards were owned at one time by the same person.
It is possible that these markings could have been from the original owner (pure speculation since both are Sweet Cap 150 Factory 30).

Here are the basic questions:
Is there any other reasonable explanation behind the "MM" other than initials?
What is the big X about? (note I recently re-sold a lot of Sweet Caps with a similar X, no initials)
Has anyone else seen this marking before?

Here's the conspiracy theory question:
If there's someone who actively collects cards with specific markings or stamps, isn't it simple to add these markings today in attempt to pass them off as the "original"?

[FrankWakefield]

Maybe not at the same "one time", but definitely by the same fellow. Maybe not a collector. Could have been an uncle who smoked, gave the cards to a nephew whose initials were M M, and the uncle marked out the cigarette brand.

[wonkaticket]

Could also be an early seller. I knew an old fellow I used to buy T206's from at a local flea market who used to mark his inventory mostly in pencil but quite a few in pen.

Cheers,

John