This stamp is perf 11x10 - no question on that. According to the Wash-Frank tables that makes it a #540. But there is offset on the back and that confuses me as 540 is a rotary stamp. What am I missing?
The perfs & design size do seem to confirm the 540 id. At a guess, while setoff (or offset) is normally associated with flat plate stamps, I suspect that this does not mean that it cannot occur for other printing methods. I have seen modern stamps where ink from one stamp has been transferred onto the gum of another stamp, albeit not through the process of stacking sheets of stamps for drying. Perhaps much the same has occurred for this stamp.
I agree with revcollector, the Type III characteristics are present, etc.
Thus the set-off must be on top of the gum. Note there are several small gum disturbances which do not appear to show any set-off. Remember, these coil waste stamps sat around for quite a while before being salvaged and sold. Easy to get slightly stuck together and have the gum pick up traces of ink from the adjacent stamps.