The groups and type reference for Meter Mail refers to the Manufacture types.
An excellent reference which forms the basis of this study can be found at
wikibooks.org
International_Postage_Meter_Stamp_Catalog/Malta
The wiki guide identifies 3 groups
1.1 GROUP A: Stamps used to frank regular mail
1.2 GROUP PO: Stamps generated by Post Office window systems
1.3 GROUP PV: Stamps generated by public self-service machines
This Study Refers to group 1.1, GROUP A of which there are 10 known types A1 to A10

The first Meter was installed in 1961