It looks like it could be an RPM. See if either of these mean anything: 1944-P RPM-001 (varietyvista.com) (top looks a bit like a worn 001) 1944-P RPM-019 (varietyvista.com) (bottom looks a bit like a worn 019) The photos posted at VV aren't super useful, but align the MM with EPU above to see if either is in the ballpark.