1535 used an aluminum housed 3 speed transmission, internally a close copy of the IH cast iron version. Despite the worry, it is a very strong design and I have proven that as I use one in my turbo diesel 1782 puller (limited Super Stock class)no failures in 5 years!
One nice feature is adjustable shift detent spring pressure, simply turn the hex plugs in to increase detent pressure.
These were made by Peerless for CCC I believe, and were also used in other applications, including a forklift! If the brackets and bolts that keep the transmission in place are monitored (so they stay tight) that is one key, if they are run loose, the housing threads strip out or break off the mounting bosses.
And yes, they do have the great fine spline axles with the HD carrier for much improved strength over the cast carrier IH used.