Hi there
Mdina was founded by Michael Harris in 1968 and he ran the concern until 1972. Michael returned to the UK to set up Isle of Wight glass. The local workers at Mdina that Michael had trained continued to produce Mdina glass and the concern is still going today.
I have checked the output of Mdina during the period 68-72 in Mark Hills excellent book on Michael Harris and can find no trace of mushrooms having been produced although Mark comments that there was much experimentation going on during this period.
So, as CHristine says, I think it likely that the mushroom is post 1972.
Best regards
Derek