How do we know:
(1) Russians actually provided all bayonets for early Chinese made SKS?
Soviet chemist, Mikhail Klochko, served as one of the technical advisors. From his own account, not only were the Chinese importing whole production lines and facilities from the USSR, they also had little to no capacity to produce the necessary spare parts to keep those new production lines running. If a given piece of equipment broke down, it didn't go back on line until the replacement parts came from Russia.
Here's a link to his article from the summer 1971 issue of
International Journal:
https://www.jstor.org/stable/40201404?read-now=1&refreqid=excelsior%3Acb6911263c5ea88ed57fcae47d6d67d8&seq=11#page_scan_tab_contentsIn June of 1960, when the Kremlin abruptly withdrew
all Soviet advisors in China, they also cut off the supply of all those critical replacement parts. And those facilities that were dependent on Soviet supply, saw either significant slowdowns in production, or were force to go off line completely.
From collected serial data, it strikes me as more than coincidental (conspicuous, even) that barely 300,000 rifles can be accounted for during the
three year period from 1959 thru 1961. That's a 60% (-ish) reduction in expected yearly production totals. Whether production slowed during this period, or there was a prolonged stoppage, this drastic plunge in production at this specific point in time suggests that sks production was one of those new production lines that was still heavily dependent on Soviet resupply for continued operation.
Admittedly, this is not in any way definitive proof that the soviets were supplying bayonets. But it is a significant intersection of circumstances.
In another way, I'm not sure it really matters where the bayonets came from. If there was a standard being followed it was a Soviet standard and
not Chinese one-- at least not in the first two or possibly three years of production-- (thus, the first deviation from the soviet design is seen in 1958 with the relocation of the stock swivel). And that standard was bland, undifferentiated, uniformity among enlisted men and the kit and equipment they were issued.
(2) there weren't more than one supplier in Russia that made bayonets for Russian SKS?
It seems very possible that blade bayonets could have come from multiple suppliers, and similar would be just as possible for other components. But I think there was still one standard. Otherwise, we could expect to see similar variations in other "original" parts-- particularly bolts and carriers.
(3) Bayonets on late (e.g. 1958) Russian SKS were not leftovers from earlier production?
At least, I am not convinced that all parts of Russian SKS were made completely 'in-house'. And I don't even fully believe that Russian letter guns were actually made at Tula.
If there is a single standard, outsourcing component production doesn't really matter. At a certain level-- everything had to be the same. Differences in knurling on bayonet handles and safety levers are insignificant. But gold or blued bayonets are a significant alteration to the overall aesthetic. And variation (read: individuality) is directly opposed to the type of bland homogeneity, and sanitized de-individualization imposed at enlisted levels of military service.