Ah. Probably right. It's illustrated on the "Cylinder Head and Bonnet" diagram and is called a "Cover, Filler." There's another "Cover, Filler" on the "Gear Housing" diagram.
The "Cooling Water System" diagram doesn't show anything that looks like a cap.
Yanmar doesn't use the word "cap" for this type of part. Not seeing any other likely candidates in the Part Catalog drawings. Don't remember ever noticing that cap on ours; I only added a very little coolant, a very few times, via the overflow bottle.
The service manual I have -- covers several models -- shows a hose coming off the thermostat cover (not a cap) marked "To radiator." The parts manual shows the thermostat cover, not no corresponding hose or cap. I'll hazard a guess that maybe the radiator cap isn't actually a Yanmar part for these small engines.
The 3TNE74 is much like the small Yanmar diesel in our garden tractor... so maybe John Deere parts people could be a source.
South River, Chesapeake Bay