Interesting ... caps are 6, 6 1/2, and 7. No idea what Durant takes a 7 in but there's been a couple on Ebay the last few years. Maybe same cap they can't get rid of. Majority of 30 - 32 are 6 1/2 of 3 designs and hub opening is a tad bigger at 6 5/8 to accept the cap but not 6 3/4. Just measured one of mine.
After market catalogues US / Canada list Hub Shell Cover for 5 bolt wire wheels 407, 612, 614, 619 and seperate Covers for 6 bolt with 3 7/8 monogram for 407, 612, 614, 617, 618, 619, E, 670 and 685. Guess they dropped the earlier 1930 2 designs with Durant across the center and other with 4 in monogram. Hub openings would be the same.
I don't know if 30 / 31 Chevy 6 bolt wires sit back on the drums but I do know the covers fit our 6 bolt hub opening. The 31 D here orig had a couple of Chevy caps in the 60's until we found Durant ones. Just as hard to find wire caps then with majoriety of 30 D's having wood spoke.
I wasn't sure I was right 1st time and went down the basement a second time and measured from one inch over on the tape to 7 5/8 before I sent the previous mail.
I just checked again and 6 5/8. This time I measured the caps as well and all three designs are not 6 1/2 but 6 9/16 almost 6 5/8 leaving very little wiggle room in the hub opening. We still call them 6 1/2 anyway.
I much prefer the earlier caps to the way the later ones are made. Metal backing is thicker and heavy galvanized and ours is thinner and less galvanized and easy rusted. The spring like holding things are differently made also on all three. Ours has a ball like thing but pointed. The Durant across center similar to the 28 / 2 6 cyl screw on's, is like ours but balls are round. The other early 30 with 4 inch monogram and rounded edges is like wavy corrugated metal no balls. Having said that I like our dished type, just wish they were better made.