I think you're a bit confused on the nomenclature. Let me try to flesh it out for you:
An SKS D model started out life as a standard Type 56 SKS and was rebuilt at a factory in China to accept AK magazines. There is nothing sheet metal about an SKS D. The rear sight ladder on a D being marked "D" is a coincidence. The Chinese utilized П, D, 3, and III as the battle sight setting on their rear sights. Which character corresponds to which factory and what timeframe the gun was built.
An SKS D does indeed have a standard size bayonet and many (if not most) will have the quick detach modification you're talking about. There have also been standard Type 56s that have shown up with these quick detatch bayonets on them. For a good read on SKS Ds (and indeed all the commercial variants produced, take a look a the D section of the Commercial page of the Chinese SKSGuide:
http://chinesesks.weebly.com/commercial.htmlYour SKS is a DP marked gun. It is a standard type 56 SKS, and in no way, shape, or form resembles an SKS D. There is much conjecture over what the "DP" actually stands for with some putting forth the hypothesis that it could mean "Deliver(y) Pakistan". There's no proof of *any* of this, and frankly I'm not a big fan of this label at all as it's a total WAG without anything substantial to back it up. We believe the
延 character stands for Yanji City in NE China as all the DP and DB guns have characters that match up exceptionally well to major cities in NE China. I only have two guns in my database, a 220027xx, 240013xx. Both numbers appear to show pretty low production numbers, but with a sample size of only two, it's impossible to say for certain how many were made during the various years. I suspect there are 23 prefixed DP延 guns out there too, but I've yet to see one. The 22 should indeed correlate to the 22nd year of production which is 1977.
Your gun originally came with a wooden stock. Any plastic was most certainly added here in the states.