You're right. Hmm. Checking Etka, it looks like only the autos have the phenolic spacers and I inadvertently stumbled on the factory difference without realising when I took them out for the manual
The 5 speed and 5HP19 use 151C and the 6 speed and 5HP24 use 151G, in which case I rescind my previous comments on using B5 / C5 mounts!