The cone clutch on the back is an added unit operated by a separate control valve, no connection to the transmission valve body. It was shifted by a separate control rod from the shift column that was moved when the lever was moved left to right in the column to change between high and low range sides. I often wondered what would happen if one put a hand control lever on it so it could be shifted on the go for split shifting!! There is quite a ratio difference, so it might have proven to be too much of a difference for split or on the go shifting.
The owner of the transmission shop that advised me to get the engine oil out of the tranny as soon as possible and put in Dexron built them in the Army and also in civilian life built the automotive version successfully for racing. It is not the weak sister it has gotten the reputation of being if correctly taken care of and with using the correct oil. I put over 20 years of heavy farm use on an M135 and never had a tranny problem during that time, thanks to following his advice.
Regards Marti