Generally, a vintage freewheel with no markings has threading that is native to the brand's country. Regina freewheels with no markings are Italian threaded. The majority of Regina freewheels would be used by the Italian industry, so not adding identifiers to freewheels with Italian threading makes the most economic sense.

The freewheel threading standard would be marked on the back on the body, not the cogs, as the cogs are interchangeable. Regina used F.F. or 2 grooves to signify French threading depending on the era. It's hard to tell in the photo, if the body has 2 grooves (French) or 3 grooves (ISO). What appears to be a 3rd groove may just be the thread start.