On the bottom of the bell housing there is a small area of material removed, probably a 1" or so for inspection and to let oil out if you have a rear main seal leak. If you had a reference from what a stock unit looked like you could compare. There might be a picture in the Bentley manual.
At any rate the DMF is steel, and nearlly all LWFW's are aluminum. The UUC flywheel is anodized blue, the Fidanza and JB Racing flywheels are bare aluminum. IIRC there is also one that is anodized purple too but can't remember who builds it. The gear ring is always steel so you have to look at the actual body of the flywheel to determine the material it's made of.
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