IIRC, they were made in the late 1970's and were part/parcel of the Savage/Stevens family of (same) slide-action/pump shotguns with various model numbers, like M-30, M-67, M-79, and others, depending upon what the decision-makers at the factory felt like doing with a particular run, and what gauge the gun was being produced in.
The "family" of pumpguns was, of course, popular - since Savage most likely made them in the millions - but nobody knows for sure exactly how many.
The reason they were so will-nilly, is that Savage/Stevens has long had a history of concealing their actual production numbers, and (ergo) profits from everybody outside the company (including the US Gov't, tax dept's, etc) - why (generally) SN records are unavailable from the factory.
This ought to be technical enough for you:
http://www.gunpartscorp.com/Manufact...3/79-41388.htm
.