From what I understand, some of the '76's were fitted with a .75" rear wheel cylinder and this is no longer manufactured. The '76 we have used the same cylinders as the '74 & '75. I don't know that the threads were different but this may be the problem?
The front calipers had a thread change from standard to metric in the '72 model year so I wouldn't put it past BL to have changed the rears especially since the TR6 was going out of production in July of '76.
Like Brian says, get ahold of John at TRF and he should be able to get it sorted out.