I'm with Pat on this, if they were added after, we'd see the occasional card without the factory number and lots of different displacements.
The question though is why there's a difference.
The back plate would have been laid out using transfers taken from a master plate with one example of that back.
I can't see many reasons to make a second master, unless the original was worn or damaged.
Another possibility is that the transfer was made as a block of backs, maybe 9 or 12 backs? That's also a bit of extra work for something that might have been needed only once.
It would be interesting to know what cards have been seen with that difference.
I have an idea or two. One being that the stone used for the Factory 30 backs was reworked by erasing the factory designation and laying down new factory designations for factory 25. That would explain it being different from the other factory 25 backs, and the slight inconsistencies between the two type one backs shown.
Another thought is that it's a similar reworking of a 350 plate to make a 350-460 plate. I think that's less likely, especially with Reulbach being one of the cards. There shouldn't have been much need to rework the factory number, unless they only had one with a different factory.
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