Just a comparison of the rubber block that sits behind the bottom of the heater box on the MGB – the one i took out of my MG last year compared to a new one. 40+ years has certainly taken its toll and no way could it be doing it’s function as a seal – although this was not the main reason the heater was not working.
See “heater horror” ……. https://mgb.tips/2020/03/21/mgb-heater-horror/
MGB tips mods and maintenance 