Hi Paul,
this could mean, that for some time both designations were used side by side, and that
there was an "EF 30" as well as an "EFo-30". But if those numbers were used chronologically,
the EF series seems to have started later, as the EF 30 was a Ju 52 design, whereas from EFo-08,
onwards there were jet aircraft amongst those designs.
BTW, in the Wagner book, the caption of the EF 50 says "from this model onwards all models were
designated only with the two letters "EF" instead of the three letters "EFo" "
So, plenty of opportunity for research, as Artie already suggested !