Peter , do you have a good photo of the bottom of the tender ? I'm a little confused ( my normal mode )
The charging port on the bottom of the tender for the proto 2 -3 volt board is a two pin port for the MTH Charger ...........the 2 batteries in the green wrap in your photos looks like my 2.4v 700mah NI-MH. that I use for my 3volt proto 2s.
..........however........
When I went to the MTH site for the engine number ( 30-1399 )on your box and tapped on the SUPPORT icon , I downloaded the manual .
On page 13 it stated that the battery was a 7 cell 8.4 volt . It showed the bottom of the tender with only the one pin port. The best info I have is that engine would have a proto 2- 5 volt board .
Possibly could be a wrong manual on the MTH site . 🤔 If you have the two pin port on the tender then I would have to say that green battery is the right one.