He was notably reluctant to put his signature on much
This is true as far as it goes but there are still plenty of large items from Mdina with his signature on. The vast majority of stuff was not signed but that still leaves many that were. This notion of not adding a signature seems more of a contemporary bit of marketing to me. Michael Harris's signature
is occasionally found on items made at Mdina and his signature
was used on marketing materials produced at the time.
I very, very, very much doubt MH would have signed anything he had not made and designed himself.
Why? Was he not proud of the glass made by his company? His signature is far
more common on IoWSG items - just look at the large number of pedestal paperweights with his signature on, or tortoiseshell squat bottles and inside out vases. He signed Azurene fish vases - not entirely all his own design.
To have made a success of two new start up companies in a brand new field I imagine took some doing, a significant achievement, he would have been aware of the potential marketing value of his name and signature and did use it to benefit his companies.
This idea that he did not sign has become over romanticised, to much reliance has been placed on the notion. It is not born out by the evidence. Very handy for collectors to perpetuate though...