the green color between the both test point is oxidation ( Grünspan ).
Manganin is a copper mixed with around 15 per cent mangan, so easy to solding.
Not the same then the tungsten mixed wires for heaters, they are horrible in connecting.
If you have a repair, I have a NOS roll Isabellenhütte Manganin 69,7 Ohms/m, made 1985 = aged.