The ones that were around previously were commissioned by someone with a Harrison lathe. You will find further discussion on the Harrison group at groups.io
A 4.5 to 3 adaptor is available from Grizzly in the US as a spare part for one of their lathes. They do not ship overseas, but if you have a contact in the US, that is an option.
https://www.grizzly.com/parts/grizzly-spindle-sleeve-mt-4.5-mt-3/p97302912
The smart money would buy a 5 to 3 adaptor, dial in a male 3MT in a four jaw and then cut only the external 4.5 MT. Less and easier work than trying to cut an internal 3MT.