quick release plate for Mamiya C 330s

... is there one with the two small pins, alongside the retaining screw, that will engage with corresponding holes in the camera base? I guess Mamiya made one originally. Otherwise, what do you people use?

I just use a regular Arca-Swiss compatible plate.
Screws on easily and keeps my C220 attached firmly to my tripod.

Mamiya did make a two piece unit - one to attach to the tripod head, and another to attach to the camera.
I also have two different Mamiya tripod heads, including the Paramender 3 head, which have the spring loaded pins built in.
The holes are also found on the RB67 and the Mamiya 645 bodies.
I use the small Manfrotto quick release plates which lack the holes but work with many Manfrotto tripod heads.
In the case of my RB67, I use one of the architectural quick release plates - the extra functionality of those stops unwanted twisting.

..thanks all. I have found a used 'original' Mamiya type with the paired pins.

For the tinkerer:

-) small Manfrotto plate with a sheet metal strip screwd onto it, protruding at both sides with bolts screwd-in at right positions
-) hexagonal Manfrotto plate modified as above
-) small Manfrotto plate in the "architecture" version (if protruding enough, otherwise modified)
-) hexagonal Manfrotto plate modified to "architecture" version