Aero O/Y Ju 52
 

Aero O/Y had five Ju 52s altogether:
OH-ALK/LAK, Ju 52/3m ce, WNr 4014
OH-ALL, Ju 52/3m ge, WNr 5494
OH-LAM, Ju 52/3m te, WNr 7256
OH-LAP, Ju 52/3m g8e, WNr 7490
OH-LAO, Ju 52/3m g8e, WNr 7493
The latter three are sometimes mentioned as -12 (7256) and as -Z-5 (7490/7493). Why?

Did they receive some modifications (e.g. different engines) at some point in their life which justified a new subtype designation?

Many thanks in advance
Gerhard