Ahh you have one of the "other than the three common types" of M21's: M21 NO
These guns are a bit of an enigma because they don't show up all that often. The few we have seen generally look like this with the following characteristics:
Known S/N between N003481 and N028811, likely started at N00001
- Long lug
- Blade bayo
- Early RSB, early FSB, early milled gas block
- Early style bolt carrier
- Side swivel stock with "NO" as an integral part of the S/N
The S/N is no help in dating these, but from the hardware, we can narrow it down to between 1961 & 1965. More than likely these were built in year 9 or 10 after the M21 N
o 900,000 series...
The 2 in front of your M21 stamp on the receiver likely indicates a heavy refurbishment in China as we see this on some early Chinese letter guns as well. Is the stock original to the carbine (side swivel, matching S/N)? If it is then I agree with Greatguns: I would lean towards stripping the paint and refinishing it and keeping it with the carbine. The Yugo stock will work, but it will definitely not be "correct" for this carbine. Alternatively, finding another long lug, side swivel Chinese stock is your next best option for both fitment and "correct" aesthetics.
Just for grins, what's the import stamp on this carbine say? Many of the early M21s were Vietnam bringbacks, but most (but not all) of these M21 N0 & M21 NA carbines were imported during the very beginning of the boom in the late 80's. Lack of an import stamp, while not a concrete indicator of a VN bringback, would nevertheless point in that general direction and is worthy of discussion in and of itself. If the carbine is a VN bringback candidate, I would think very carefully about how to best preserve it & not damage it further.