The motor units are Robbe EF76 with fitted gearboxes, rated at 6 - 8volts, superb units and very reliable, did you strip and clean the gearboxes as well???? were the photos taken before you cleaned the motors???
The battery space looks to have been fitted with 4 x 6v 4ah batteries, probably giving 6v drive power, might have been 12v. is there an ESC fitted?? that would give us the final key, apart from the red and black wire in the 4th photo is there any other wiring???

Mark

Regarding the batteries, I suspect it was 2 x 6V ?Ahr gel cells. One for each motor. 2 x ESC and possibly the Robbe mixer.

I agree with Jarvo it could have been 12v from 2 x 6v in series but it was common to use 6v during this period.

Could be the space was for the ESC.