Stumpkiller, Musketman...I got my hands on that 28 ga. and can give you a little more info: it has long side plates, so it is not a box lock. It has a 6 digit serial # on the bbls, on the shelf, and on the metal of the forestock. The forestock is "dainty", and narrower than the bbls by quite a bit. Going over the marks without 2 oz of Elijah Craig in me, I can describe these marks: left bbl, underside, a tiny 5 pointed star, like an asterisk above the capital letter "T"..about the size of a typewriter cap. T;
above the word "choke" and "137", then, in a circle, AG above the letter "M"; The underside of the right bbl is the same, except that before (towards the muzzle direction) *T is the inscription "MAG" with maybe "I" following it..not as clear as to whether that is a letter or an incising...I think a letter. Each bbl has, almost at the breech, an oval containing a crown above the letters "ML" and that little star I describe above.
In further looking at the gun, I would speculate that it has had a more modern buttplate retrofitted...the material appears newer, and is some kind of black rubber ((?) material...looks like buttplates that came on guns in the 1950's...finally, it does not have a straight 'shotgun' stock, but has a semi pistol grip...a swelling...original, I'm sure. The latch that holds the bbls in battery has a visible transvers bar that goes through an extension of the top rib, and which can be seen from the "port" side, leaves a gap there when opened, a visible filled gap when closed...
That's as much as I can make out...if I get around to buying a digital camera as my wife keeps asking me to do, I'll put photo's on, until then, I'd like your, and the rest of the fraternity's, opinions..Hank