The 1900 have two opposed dovetailed or, if you prefer, beveled bolt lugs, the case head is partly enclosed in the bolt head and the extractor is a small claw, plus the ejector is a sping loaded plunger embedded in the head (like most "modern" push feed actions).
There is no additional third lug close to the root of the bolt handle, instead, it's the bolt handle that acts like a third lug.
The 1600 and have a long, massive non-rotating blade extractor, two opposed lugs located on the bolt head and one close to the bolt handle root, a slot cut out for the blade ejector in the bolt head.
A large ring action (which both are not) receiver measures 1.410" wide at the front receiver ring (where the bolt locks) while a Small Ring action measures 1.300" at the same place. Both HVA 1600 and the 1900 HVA/FFV/Carl Gustav are of the small ring design.
The 1900 is a modernized 1600 and usually can be fitted in a 1600 stock with some work. Both used a similar stock design made of pale walnut with Monte-Carlo comb and cheek piece, with a squarish fore end nose made of rosewood.