I had a recent experience in another hobby that has made me reconsider my thoughts about offset transfers on T206s. Those are the backwards inpressions of the backs that you sometimes see on the fronts and the less common backwards fronts you sometimes see on the backs.
I recently bought a couple card proof versions of stamps from a seller in the UK. These are on a cardboard that's similar to the cardboard used for T206s, maybe a bit thinner, and uncoated on both sides to more readily accept the ink from the engraved plates.
Both were shippped in a small glassine envelope, with stiffeners inside a regular envelope. When I opened them up to my surprise the 6c Post office dept had left a nice offset transfer on the back of the 6c Justice dept!
The last printings of these were I think 1881 , maybe as late as the 1890's. But for sure no later than 1900. So this happened 110-130 or more years after printing. And I'm positive the transfer is new since the sellers description had nice clear scans of both front and back.
In the past I believed that the offset transfers had to have happened during production, or shortly after unless some chemical was involved. I'm not so sure now. The very clear and complete ones probably were during production, but I now believe the lighter ones especially in black may have happened much later due to stacking and pressure. And if the black ink was slow to dry, the others may have been slow as well.
Steve B