The steel framed 31s and early Wingmaster 16ga guns aren't that good. It always amazes me anyone would pick those over old vintage Ithaca 37 pump guns. By old vintage I mean ribless barrels, fixed choke and 2&3/4" shell capable receiver.
I suspect the 'L' designation on alloy framed Model 31s is myth propagated by guys with Remington books and old catalogs.
PJ, what in heavens name is wrong with a steel framed model 31 or an early 870 in 16 gauge? That may be the first time I've ever seen that comment on the internet that Al Gore invented! Also, you are quite wrong about the letter L designation-some have it, some don't. I've seem them both ways.
There was a bit of a war going on, you know. Remington had a lot of things to sort out just prior, during, and just after that war-perhaps hunting guns for civilians, and what was marked on them, were not a priority for a few years.