Looks like you have the split image screen. There is one with a larger microprism grid in the center which may be more like what the Mamiya has. But, as Matt says, you really need the viewfinder hood to accomplish much with any screen. FWIW, when using a WLF camera I often do most of my composing with the magnifier in place, whether the camera is a Hasselblad, Rollei or C330.
I have some old brochures that will have the model numbers of the older screens, I'm not sure where they are right now, but I'll work on digging one up.
If the image isn't bright enough for you when you get your finder, you may want to consider one of the acute matte screens, I haven't used one, but lots of people like them and prefer them to the "traditional" screens.