I've never heard the term "manufacturers mule" used in respect to firearms (or anythingelse). However wikipedia list a similar term as one from the automotive industry

"A development mule (test mule, or simply mule) in the automotive industry is a testbed vehicle equipped with prototype components requiring evaluation. They are often camouflaged to deceive competitors and thwart a curious automotive press."

Probably what the auction blurb was alluding to.