http://vssmb.blogspot.com/search/label/identifying
Rain has some pretty good tips for identifying machines on his blog. The 15-91 and the 201-2 are about the easiest of the old black Singers to recognize when you see them, as least for me. Any black Singer machine that has the tension assembly on the face plate instead of the front of the machine is a 15, and if it has the potted motor it's a 15-91. The 201-2 can be spotted at a glance because it's the only full size black machine that has the light fixture directly on the front of the machine. Also the tension is on the front, it has the potted motor, and the switch for the light is white and on the front where the cords plug into the machine.
I know there's a book about the history of Singer at the library but I don't know if it has the info you're looking for. I think the old Needlebar site had a chart for identifying Singer machines. I think most of us have learned by experience just like you are.
Cari
I know of no reference books or charts for foreign brands.