There is currently a very rare intermediate detachable magazine SKS up for sale on Gunbroker. I don't like to post links to open auctions as I'm not promoting anything but the title says:
"Rare Chinese Norinco 63 sks 56 matching rifle thumbhole stock notM vintage"
The seller has also put ample pictures of the rifle which would answer just about every question anyone might have about this type of rifle.
There has been some discussion about this type of rifle recently and some members have attempted at some point to make replicas of them. As I recall these rifles only came in for one year and were precursors to the SKS-M's. I only ever saw one of these in person back in the day. These are essentially SKS-D's with 20" barrels that were put into newly designed thumbhole stocks. This style of stock was later used in somewhat modified form (with black "truckbed liner" inserts) for the SKS-M's that came into the country in 1994. The stock on these rifles was pure wood with no reinforcing inserts or black portions (unless you count the recoil pad at the back). The other significant difference between these rifles and other models is the front sight group, which has the bayonet lug removed (not replaced with the spring loaded rod retainer) and the top of the front sight hood cut off giving it the appearance of European AK rifles.
I'm thinking the seller may be on this board, but if not, I'm sure all of you would be interested in seeing it for comparison sake.