S'funny, I don't have a problem with the original mirrors on my '76 (at least not now that I have a replacement lower section on the driver's door courtesy of one of our number
).
Steve, I think you'll find there is a 'key' on the upper stem that needs to pass through a gap in the plastic/nylon bush (or you need to remove the bush to refit for your new upper). You'll see what I mean if you carefully lift the bush out of the lower arm.
Cheers,
