Then look here; http://www.marlinowners.com/forum/marlin-years-manufacture/31126-list-336-model-differences.htmlLook on the barrel--the model designation should be stamped there along with the caliber and where the rifle was manufactured.
I'm unclear who you mean by his. Is that your son's or the man who has the gun for sale? You say "there are no model #s. If there are no model numbers, how do you know it's a 336?It has the "JM" stamp on it, but the guy who has it says there are no model #s on it. I looked at my son's who has a 336 AS and I didn't see the "AS" stamped on his.