Deck Pump-Out Fitting (weird thread)

[HJ51]

I have a 1992 Com-Pac (CP27) and every time I come in to a marina for a pump out they have trouble finding an attachment that fits the threads of my deck fitting.  Usually they never do and I'm either out of luck or they pump out without it being fully screwed in which could be disastrous if you know what I mean.

Anyone know what the diameter and thread size would be for an adapter?  Better yet have a link to where to I can buy one?  Thanks in advance.