If so, my guess is that it is a put together scope or maybe a depot rebuild. If "Telescope M73b1 is electropenciled on the tube then that would indicate the tube is from an earlier version scope. That one looks like it should have the roll stamped "Serial No." on the side and possibly the drwg number. I have never seen a parkerized M73b1 on an A4, $500 would be beyond what I would pay for it.
Serial # 8328..Scope is a mixmaster/parts from "who knows"?? - I have reports of two other scopes with parkerized tubes; again "who knows?".