Curious About Green Marks Alongside Cancels. (Grease Pencil?)

I'm going through a huge bin of stamps, with thousands of the 3c Washington Scott #720.

I'm curious about the green marks by the wavy cancels. Since these were mailed a few days apart from different post offices, I'm guessing they were marked up by the recipient mail room.

Because they almost mimic the cancel, made me curious if this was something the post office did for some reason... or maybe just in post offices in Nebraska. Probably a silly question with the only answer being "who knows?" Although there is some very obscure knowledge here...

Shame, these are not full covers. Harder to interpertate their meaning out of full cover context.

I agree with the comments so far that it is often extremely difficult to tease-out much postal history from small clippings or soaked stamps. I don't recall seeing such marks before.

That said, they do not "speak" to me as postal in origin, but rather the similarity of the envelope paper, the same position of a pair of the same stamps, the closeness of the dates of the cancels (and from the same state), gives an appearence of two prepaid reply envelopes sent out to customers to return insurance of other legal forms which were known in advance to go into the second ounce. And the green marking being internal clerk/secretary marks noting some level of processing at the receiving end. But of course seeing a full cover might shoot down this theory entirely.

Thanks John. That's a geat analysis based on experience, just what I was hoping for.