What is different between the two flues from 1975 and 1986?

Could it be that the flue was found to be not performing with plaster,
so the standard was subsequently changed but the flues remained the
same?

Just because the flue [materials] met a standard, does not mean that
the construction and subsequent plastering conformed too.

How do you know that the flue meets the relevent standards anyway?

I have never seen, nor known plaster on a flue block not to crack. They
should always be boarded

dg