Looks like a nice clean earlier 870. I agree about it maybe being a 28" or maybe 30. Might just be the picture angle though.
Scarce?,,yes I'd say it would be scare when compared to the number you'd see around in 12ga or even 20ga in most areas.
But that doesn't convert to rarity & extra monetary value necessarily.
16ga guns are usually the last to sell on the used gun rack of common repeaters. A drawback for many people is the price of shells. The sportinggoods and big box stores don't often have the 16ga shells at sale prices like $5/box as they do the 12 and 20ga ammo. Once in a while they do though,,grab them when you see 'em.
In actual use,,the 12,16 and 20 all account for themselves at about the same level. An ounce of shot is an ounce of shot. They all launch em at the same 1200fps +/- depending on the specific load..