Thanks again, guys! For future lurkers, here's what I've found after a bit of Google homework...

It seems that "Rectus 21" is a company and product line that's become a defacto standard as the "mini industrial coupler". The company itself is Germany-based and only offers metric and BSP threads, but many other companies offer "21 series" compatible parts that are NPT threaded.

The "KA" part denotes features of the female coupler - single or double valved, etc. KA seems to be the simplest female coupler, and works analogous to the larger styles you'd buy off the shelf in a hardware store.

Cheers,
Richard