FrenchBern et al,
I was referring to the early units IE up to about 1977 or so. There was no electronic series regulator on the early units; voltage regulation was by the transformer design, and you are correct; pretty much like constant voltage up to transformer saturation which was the current limiter but the transformer design gave stable output voltage with varying input voltage.
The Magnetec manufactured 1980's vintage unit I have in the shop lacks voltage regulation, just a center tap voltage doubler like rae described with a very small capacitor for the filtered 2 amp max rated output with the other circuits unfiltered. The constant current reference was to older industrial chargers like mercury arc rectifiers with multi-tap transformers, not used for RV's as far as I know.