John,

I haven't seen a real monkeyface for sale in a very long time, either. This image shows one that terminates the armor sheath on BX cable. [Linked Image]
The basic function is that of a weatherhead, that is, end the raceway or cable and let the individual conductors run on in open air.

If one were to transition now from NM to K&T, using contemporary material, I'd suggest using a threaded ½" weatherhead with a ½" threaded romex clamp.

Al

Al Hildenbrand