I picked the following off of several B-29 engineering drawings:
8 oz O.D. Duck - spec 16092-B
Fire, Water, & Weather Resistant 8 oz O.D. Cotton Duck, spec CCC-D-746 Type II
Spec 16094 Type II Army Duck, Treated, Class A, 8 oz
A search for spec CCC-D-746 shows it was cancelled on Jan. 1, 1960. Searches on 16092 and 16094 yielded no results. So the Duck of choice seems to be the 8 oz form.