I think they are on the paper catalog they mail out [coupon codes). I have the latest one will take a look.
I used to have two century CETMEs. The HK G3 mags will fit, and lock, but typically fit loose. Becasue Century made the receivers on the CETMEs, no two are exactly the same so it is a crap shoot to get one to feed reliably with the G3 mags. Mine were both so unreliable I could use only CETME steel mags, and even then I had to tweak the feed lips to get every round to strip, my receivers were so out of spec.
The CETME mags are not HK, They were only made by CETME, and are steel only with a slight curve to them. All G3 mags are straight, steel or the Para aluminum waffles. Tapco used to have a ton of them but don't know anymore since I sold both mine.
Hopefully you got lucky. If you have a CETME, you must know about
Cetmerifles.com right?
They can answer any question you have about it. Most memebers of that forum are fiercely loyal to their cetmes, and to working out the many issues they have. Good site.
The G3 mags are dirt cheap so you can try it to see if it works. Don't be surprised if it doesn't feed reliably from the G3 mags though.