Mark,
Correct, they are anodised aluminium. /ubbthreads/images/graemlins/cool.gif
These were fitted by my engine builder Mike Garrett of Garrett Machine in Lebanon Oregon who has stocks.
Mike also made the timing cover and has forgotten more about Ford engines than the rest know.
I asked him to tap the block. The rest is blueprinted.
The Boss block is a later and lighter addition for production cars. This one has stronger webs.
This block was made by Ford Experimental at Dearborne and was designed purely for the GT40 racing programme, hence the 4 bolt mains. It was mated to Gurney Weslake heads.
I now have a second, full Gurney Weslake engine which I will not corrupt by adding screwed plugs.