Not sure which seals you mean, but from the attached first photo (I don't recall where this photo came from, so apologies for not giving the owner the recognition they deserve):
A: On my cab, this was a rubber 'P' section. But I've also seen a double beaded canvas, similar to that found on the top of the grille on a flat fender jeep.
B: Only ever seen this as the double beaded canvas as described in 'A'.
C: On my cab, this was a double beaded canvas as described in 'A'. But on an NOS front shell I picked up a while back, this was a rubber 'P' section (see second & third photos).
On page 154 of the January 1941 Chevrolet parts list, this 'anti-squeak' is listed as webbing (part no. 114286) or rubber (37852 R), so I guess both were used; strangely, only rubber is listed for the C15A. Not sure about Fords, but I assume similar?
Hope the above helps.
Owen.