I've never seen a "marked" shift knob on an aux transmission. Usually in a big truck it was just a plaque on the dash. Myself, I just wrote it in sharpie so I don't forget LOL!! Though it shifts the same as my Mack compound, but has 4 spds instead of only 3 in my Triplex. Macks always have a big plaque above the windshield on B models, later trucks were on the dash.
Yes, the 4 spds are not a common car pattern. You shift it in a big "U" pattern.
Lo lo OD
Lo direct
I put a Hurst T handle on mine! Where my Mack sticks are just plain bakelite knobs.
Here is the piston grip I started with, but it wasn't comfortable to use at the angle it was so I swapped for a T handle: