Its always possible you're right, but why not measure anyway?
What if you get lucky and discover what I'm suggesting is [possibly] right. It will save you some wear and tear! Your claim there was a unique 16-size M870 is the first I have ever [EVER] heard about.
I have followed closely the developement and marketing of the M870 family of pumpguns since the 1970's.
The only lightweight-framed Remy shotguns I'm aware of and built by Remington started about the same time Big Green made (for a few years - mid late 1970's and early 80's) their 20 gauge (and 12ga) "Special Field" M1100's and M870's.
Also prior to and during that time one had to stipulate when buying a M870 whether he wanted a standard 2 3/4in gun, or a 3in magnum. Before the introduction of the "Express" versions of the M870's, Remington didn't offer all 870's with 3inch chambers, and charged custiomers extra for any kind of "magnum."
I know all this as I was an over-enthusiastic high school student during those years, (growing up in florida) who tried like hell to be a successful duch hunter with a couple 2 3/4in, 12 and 20gauges pumpguns. Florida was a lousy "duck state!" and I didn't kill very many.
Since those years Remington has always offered small-frame M870's, but prior to then, I was only aware that Browning had made a (Belgian) 'Sweet Sixteen' version and size of their famous A-5 semi-auto SG, and that Ithaca had made their M37 pumpgun on a 16ga frame.
Also Winchester had built a 16ga-framed Model 12 pumpgun (smaller than the 12, but bigger than the 20gagg).
That last point about the 16ga M12 I know is correct because a buddy had one, and is the only Model 12 I've ever actually shot. It also the only 16ga I've actually shot!
Good luck, Boyd!