Having studied this problem at some length, I believe that Kimball had some kind of a jig or machine for making the inner (vacuum) valve seat, which was then glued into the stack. This seat was further sealed into the stack with washer of tan pneumatic leather to insure that it was 100% air-tight.

    These series of pictures basically show how the valve seats are put together. What's interesting to note is that there wasn't any glue on the felt donut.

    To get the red fibre washer off the stack, I first scraped off the tan pneumatic leather that was used to seal the washer to the stack. Then I applied high dry heat from a clothes iron for about 20 seconds. This softened the rubber on the cloth which allowed the fibre washer to be removed without damage to the washer. However, as you can see, even though I tried to quickly remove the cloth from the washer while it was still quite warm, some of the material from the washer stuck to the cloth.

    Considering that rubberized cotton-backed thin pneumatic cloth is not currently available in the US, how one would go about rebuilding these valve seats today alludes me. The material would have to be soft, pliable, 100% air-tight, and it would have to conform to the felt donut without puckering.

    Lastly, I measure the thicknesses of the three materials. The red fibre washer is 0.029". The felt donut (washer) is 0.040". The cloth is 0.008".