I read someplace these are just rubber. The originals at least on the bs34 are cloth covered with rubber, which is typical way of making things like this that need to last awhile.
That's right, on my 38's they're rubber coated cloth. On old H*ndas same story. Fixed a couple vacuum diaphrams (dashpots) on my cousin's 87 Civic using that Napa Plastidip spray, the cloth base gave it good footing.
Got a couple of JB diaphrams waiting to go on, they look, feel, and are reported to not have cloth inner, uses new/different rubber material, I guess doesn't need the cloth...