Weiler Commodor 80NC gearbox implementation / control

Hi all, I've acuired an old Weiler Commodor 80NC lathe a few years back. The old controller already broke while in operation at the previous owner. They then used some hacks to get the servos working, installed a DRO and used it as a manual lathe.
 

I would like to get the lathe back to working order using linuxcnc but the lathe has an unusial gearbox. There is dahlander motor coupled with a two speed dual clutch gearbox driving the spindle. Inside the spindle housing there is an additional manual gearbox.



The original cnc-control used the commands S1, S2, S3 and S4 in the cnc-program to control the 4 different speeds of the dahlander motor + gearbox combination. The spindle speed itself is then determined by the setting of the gearbox in the spindle housing. So the gearbox determines a range of speeds that can be programmed in the cnc program.


There is only a rotary encoder + proximity switch connected to a seperate gearbox shaft, so i guess there is no way to just check the gearbox position without adding additional sensors.
 

Does anyone have a suggestion on how to control this spindle in linuxcnc?