Not sure, but that may be an aftermarket replacement for the 660 style aluminum housing. I have seen some other cast iron ones in non-GM aftermarket boxes with similar long casting numbers.
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Jeff Helms
65 Z16 Survivor
65 Z16 drag car
66 Chevelle L78 unrestored
67 Chevelle L78 unrestored
67 Camaro SS350 Survivor
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