Very Scarce? US #17 Used Stamp With M.o.b. Stamped On The Back

I have a US #17 used stamp that has "M.O.B." (Money Order Broker)stamped in purple on the back. Just curious if you have ever seen this before. Check out the pics.

probably an owner's handstamp

Could it be "signed" by whoever printed it? I've seen a few stamps similar? Would it be best if I got an expert cert?

No. This is not signed by the printer. This is an owners mark. It's a nice #17, if it had a full margin at the top I'd say it would be worth getting a cert for it if it's otherwise sound, as it will improve the value, but with the top margin cut into it's just a bit diminished. It is a lovely 17 though.
The main thing is to protect it from humidity so that that purple hand stamp doesn't bleed through over time. Keep that stamp dry.