I can't quite figure out this cover. It appears to be a very late use of the 10 cent Columbian and Scott 303 with a Saltsburg PA MOB postmark used as a cancellation, from 1922 to England via registered mail, with some nice reverse postal marks as well. I think MOB is Money Order Business - I guess to indicate a money order? I did not know they had special postmarks for this.
Maybe this was a payment from one stamp collector to another? I'm not sure why a business would be using a stamp from 1893 in 1922.