This Megger is one that is made to measure milli and micro ohms. It uses a 2A current for this. The meter part uses a ICL7107, a multimeter IC that is around for many years. It is f.i. used in the Fluke 8020. It was dead but not a problem because they still make it.
When I can not find service documentation I must make my own schematic. For something like this it is easy. I make 2 pictures, fool around with them in the Gimp and this is the result: