I've only seen them made out of aluminum...
037-121-144H picture 
The one I got from Germanautparts.com ($4.41 hose flange, upper rad hose) looks just like yours but has a different p/n. I found it over the phone by talking to the sales rep. It is not in their online catalog (per the rep) since it's in short supply. It's made by Meyle p/n 068.121.132
The bolt holes has metal bushing inserts so you can't really over tighten and bend or crack the plastic. The o-ring sits in a recess on the flange so I do not understand the comments about sanding it flat???
Yeah, if its got an Oring, sanding will not work. O-rings rely on the squish in the groove. NASA learned about this the hard way ;-) Mine were always aluminum and never leaked (stock gasket with a thin layer of Permitex or RTV on both sides)