I tested a bunch of the fittings I have lying around to see what would fit. Of all I found only two that would not. One is the Koolance 90 degree rotary and the other was a 3/8 BPST fitting for a TC rad LOL. I tried to get it in though.
The roundup:

Koolance fittings:

BP/DD fittings:


DD fittings:

Others (think one is an EK but not sure):