I think it's a 15-90 because of the belt on the handwheel, these machines are easy to distinguish because of the face plate/nose plate and the potted motor on the backside.
I have a 15-91 with no belt so I don't have to worry on belts being worn down.
You don't have to mess with the tension too much with these machines, they make an awesome perfect stitch, weighs quite heavy it's best to have it in a cabinet or one of those carrying plastic compartments that sell at WalMart; but you can still sew using them just the way it's on your photo.
I ordered a cabinet specifically for my machine from Ebay.