The 1640 series is a modified small ring commercial action, usually referred to as an HVA action, that replaced the large ring 640 made by FN.
The 640 series were made on both "strengthened" SR 1896 style action, strengthened by omitting the thumb slot in the left receiver rail, and LR actions made by FN. Another one of those nomenclature issues where two different items have the same name.
Simpson LTD has a lot of 640s built on both actions.
