I would appreciate some help in identifying this impressed rectangular mark

The mark looks very much like that of the British potter Malcolm Pepper (1937-1980). His work was much admired in Britain but he was outside the general British Leach-Japanese school. Although his work looks very much in that style. He died at a very young age of 43, just before he had a big exhibition which established his reputation. A Google image search will give samples of his work in Britain.
             It may not be of course. It would be very unusual to see his works in Arizona! We also have a short thread on him on here. Search Malcom Pepper in the box

I appreciate you weighing in. The mark does resemble Malcolm Pepper but not on the angle as you said.
I will do more research - but thanks so much for the responses!