You REALLY do not know the vehicle at all do you?
There are two cables on the lever - the lower cable is for the parking brake and the upper cable is the reverse lock out cable. Whoever you bought it from should have explained the operation, but from the looks of it, it needs to be taken to a shop for the routine maintenance that it has never had.
If it is adjusted and working correctly, you park the vehicle, lock the brake and it prevents it from moving unexpectedly. Now the upper part is the reverse lock out - you are supposed to press the plunger down, applying the brake pulls the cable to release the shift drum so it can be shifted into reverse. BTW, it appears the parking brake lock lever is bent and needs to be replaced or at least reshaped into a functional operator.
Note: the rear brake operator is a foot lever on the RH side - adjusted correctly you apply the rear brake by foot before setting and releasing the parking brake hand lever.
Good luck