Very interesting, thanks for the post and pictures. I remember my days with an M 135 towing heavy loads and plowing snow. foot on the floor a lot, working the transmission shift lever between hilly and level to keep upshifts from happening too soon. That engine and transmission took a beating and kept on working fine but I kept up good maintenance and used good sense when driving it. I changed out the engine oil in the transmission, replacing it with Dexron ATF at the advice of a tech who worked on them and had no failures. Slow speed, hills slower! When we went to multifuels, especially turned up, what a difference! We have had excellent luck with them for years.
I hope your place continues to give you much enjoyment!
Regards Martin