What Are These Margin Handstamps On Plate Blocks?

Are these some sort of inventory control number? Were they done by the USPS, a collector, or?

revenuecollector Just guessing but I have several plate blocks and none of them have these extra numbers on them so I would say they were either put there by a collector or by a seller but definitely not by USPS

Looks like the numbers generated by one of those mechanical numbering machines.

Someone could have used it for inventory purposes (i.e. the number of panes they had for any one issue). I agree that it was probably not the USPOD as they would not have provided such a device for local post office use, however, I don't know if that would have prevented some small post office from using one anyway. However, it still seems a more likely scenario the number was affixed privately. In fact, back in the generation of those stamps, it may have even been a business that wanted to use commemorative stamps on their mail (before postage meters came into use by many small businesses), so it could have resulted from that.

It seems to me that collectors and/or dealers would not have wanted to deface the look of their plate blocks by using a numbering device, so it more likely is from a company's inventory control system than from a dealer/collector.

Of course, this is all just guesswork on my part.

I think that whatever it is, it was done by the same person at the same location with the same brand of rubber stamp as the 6's are identical.

I think they are ink inspection numbers