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D90's in 9's only cam on Turbos and were forged. Quite rare. The 8's are much more common.
The 16x9 ET52 (52.3 if you want to get technical) D90 was the standard rear wheel on the '90-91 928GT, were optional on the '91 928S4, and they were cast. Part number 928.362.118.02 Relatively speaking, these aren't too difficult to find.
The very similar looking Club Sport wheel on the '89 928GT (and 951) was a 16x9 ET60 and was indeed forged. Part number 928.362.119.30 Relatively speaking, these are very difficult to find.