Good morning Peter! Thank you for that. I can only find MOBs marked on receipts or envelopes. I read that if one was used on a stamp it was in error of the postman. Do you think this was done intentionally at the request of a collector?
The MOB devices were intended for use on Money Order documents, but were occasionally used on ordinary mail. The postmaster or clerk simply grabbed the most convenient device and used it to cancel the letter.
Here is one example.
I agree with John Becker that the example from Bomarton was probably nothing more than a favor cancel.
Here's an improper use from Wellington, WA on a postcard, c1925-28.
Here's one from Norland, VA, It may be in that gray area for some collectors. Going to an insurance business, I can see a scenario where the customer bought a money order and immediately put it in the reply envelope and handed it back to the window clerk, who used the same MOB handstamp just used to process the money order forms.