Yes NATO STK No (NSN) mate, and the 100rd mag NSN should work, as we have NSNs with the 21 (Canada) designator for some Minimi stuff, and it works fo us. I can't see why Canada would have thie own NSN for something that already has one in the system.
If your 200 rd mag is OD, then the AUSCAM 200 rd mags might not be in your system.
The second group of two numbers in the NSN is the country designator. Examples, 21 Canada, 00 USA, 01 USA, 02 USA, 66 Australia, 99 UK, 13 Belgium, 12 Germany. Thats off the top of my head, as each NATO country has one. Hope I don't confuse ya.
Cheers,
Wes