I stumbled across a souvenir sheet issued by the Sovereign Order of Malta. Scott does not list them but saw some ebay lots with some catalog number. Is there any leading authority for these issues?
That's correct, but the sovereign order of Malta is not the same as the Republic of Malta ( or is it still a commonwealth monarchy with Elizabeth as head of state? I have no idea. Wikipedia probably knows)
I read that these issues are generally regarded as cinderellas.
This is a link that shows issues from 1966 to 2018. Some "emissione" number are given for the earlier years.
The Sovereign Order of Malta is an organization based in the Vatican. It is independent of the Vatican, and has postal treaties with many countries. It should be recognized (my opinion) by Scott and other catalogs since the stamps are valid and used for international mailing, but this is one of those quirks of Scott.