They're "offset transfers" With ink that isn't quite dry and enough pressure the wet ink on the front will transfer to the back whatever is on top of it.
I used to be pretty impressed with them, and they're still impressive, but I had a 100+ year old item printed in black on a similar cardstock to T206s transfer to another item while in the mail between the UK and US. I've learned a bit more about the formulation of inks back then, especially black.
It's possible these and the Tolstoi transfers didn't happen during production.
Are they all P150? if they are that would be pretty convincing that it was during production.
Steve B
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