In article ,
Martin Angove writes:

It is important however to realise the difference between GU10 and GZ10
laps which otherwise look similar and will fit the same connector bases.

They don't have same bases -- full reflector base has a bevel round the
bottom which stops dichroic lamps being fitted. Substituting the other
way round is OK. This is specifically to stop a dichroic lamp being
used in a fitting not designed to take the heat it dumps backwards.

--
Andrew Gabriel