Allow me to be the first to say that this post (mine, not ours) is probably totally worthless, except that I though it might jog someone else's memory. Anyway, your description and pictures reminds me of a product that 3M used to make, and I might be thinking back 30 years ago, so don't know if it's still available. It was strips, maybe 10-12 long by maybe 3/8 wide by 1/4 thick, came in a box something like 1-1/2 by 6 by 10-12, and the strips were laid on cellophane or whatever, stacked 4-6 deep. As I recall, it was for automotive and RV windows, and you would press the strips into the frame and then press the window into the strips. The strips were soft and pliable, so you could shape or rip them to length with your fingers, but they held their shape until compressed, not runny like tar or whatever, and I think they came in a light gray and a charcoal type gray. Again, can't be any more help than that, but that's sure what it reminds me of seeing.

Couple high angle shots from attic looking down onto sill: Thanks.