That wasn't the only one.
Chinese SKS 7.62 assault rifle,
SKS is not an assault rifle, nor even an assault weapon. Aside the bayonet lug, it's a very plain vanilla looking semi-auto carbine fireing an intermediate power catridge, about the ballistic equal of the venerable 30-30 lever action deer rifle catridge (yeh I know their are bolt action 30-30s too, probably break action and maybe falling block too for all I know, but it's most commonly associated with "cowboy style" lever actions)
The SKS, in it's original and most often seen form, doesn't even have removable magazine, one of the requirements to be an "assault weapon". It has a wood one piece stock, just like mnay hunting rifles of old.(aside from the gas tube cover that is) althoguh it can of course be equiped with a more modern plastic or composite stock.
The SKS is 7.62x39, not NATO 7.62 (which would be a .308 by our standards). I currently own several SKS, and previously owned a .308 (still regret getting rid of it) - there is a significant difference in size.
You are correct that most SKS are fixed magazine, with wood stocks, and that you can buy after market POLY stocks for them (currently have two POLY stocks)- they cost $60-80 on average.
I know most people dont care for the SKS, but they are affordable, easy to work on, fairly cheap to shoot, not too powerful with recoil, and all around decent little rifles. Although, I have read that they are not the most accurate things, especially at distance.
On the bright side, they are incredibly durable.
It is in a historical and technical sense, but probably not a legal sense - depending on how many 'evil doodads' are attached to it.
Mid-powered cartridge (7.62x39 Russian), high sustained rate of fire, etc., all conform to the concept of the 'assault rifle', and the SKS compares well to the best of the breed.
It is considered obsolete because of it's stripper-clip, fixed magazine -- but many shooters would rather use one for say, hunting deer, than an AK style rifle.