I have several MH-25 and MH-25a chargers, so I did a little research to reconstruct what charger was coming with what camera, and I would like to share my results:
Cameras in D7xxx series: up to D7100 had the MH-25, from D7200 onwards they have the MH-25a
Cameras in D6xx series: up to D610 (all models sofar) had the MH-25
Cameras in D7xx series: (D700 had a different battery) from D750 onwards they have the MH-25a
Cameras in D8xx series: up to D800E had the MH-25, from D810 onwards they have the MH-25a
Cameras in Zx series: all models sofar have the MH-25a
Cameras in 1Vx series: model 1V1 had the MH-25, from 1V2 onwards had different batteries (and chargers)
Naturally, as stated previously, the two charger models are interchangeable. I have not done any research regarding the charging speed and the amount of charge given to the various battery flavors (15, 15a, 15b, 15c), but I haven't found yet a significant difference needing further investigation. Lithium-ion charging times are so much shorter with respect to previous NiMH batteries!
Ciao from Massimo