this is from the MK3 setup right?
No. I call it a suction plate. It's basically the back side of the MK1 Sanden compressor. It's round (like the compressor), and comes off when you remove the bolts that hold it on to the compressor. It's where the high and low pressure hoses mount to the compressor. I'll try to get a pic later.
Anyway, 90% of the used diesel compressors I see are toasted, and it' takes some specific tricks to test and see if they're good, so I just figured I'd try to get the plate. I figure someone has to have a couple of used MK1 compressors with seized bearings or internals.