Actually, if you luck out with the late model DG3804 (good chance), do not waste your time on ETA movements for slow beat and having to deal with overlays.
The new DG's datewheel's, if you can live with the font, that IMHO look like plausible service replacements, line up near perfect with Gen dials.
So to recap 21J's (most likely a DG3804 in a GMT):
1.) Datewheel lines up with gen dial's
2.) Has slower beat.
~ Trying to find a ETA 2846 worth a $hit, then servicing, then dealing with overlays, is probably not worth the effort these days. ~
ALL GEN except the DG3804 and hands: