I'm not sure about that Bob, but I'd like to comment on a new set of 19" tubes I bought from Walt Bratton for our '30 Roadster, two years ago.
I was in the garage just yesterday. Car is stored in a heated garage.
I checked the air pressure and three were still on 35 pounds the other was at 33 pounds.
Two years and I've never put air in them
THANK YOU Bratton's for supplying good inner tubes!