John, no neccessity that they have really switched intentionally. Or if so they may have figured that there is no other factory 21 (Hungary) anymore from which they may have tried to be distinguished from.
Anyhow there is a 7.62x39 with “21 09” hs. That is surprising in itself as already in 2008 there was a hs which followed the pattern which was used since the NATO calibers got adopted where the caliber is given in the hs. Means there is a “7.62x39 21 08” and a “7.62x39 21 11”.
So again it seems that there is no rule without exception.
For the records: so far I have never seen a Polish 9x18 with caliber designation in the hs.
All of the above of course applies only to military headstamps.