The most common manufacturers of auxiliary transmissions were Brown-Lipe ("Brownie") and Spicer. Those companies eventually merged, I believe. Timken also made some auxiliaries. Way back in the dark ages, a company named "Truxtell" made transmissions and 2 speed rear axles for Ford Model T, Model A, and Chevy called a "Tip-Toe Matic".
The Brownie style driveline mounted transmissions are all "crash box" types, no synchronization. It takes a lot of practice to learn to shift one without some serious gear grinding. The two shift rails in your photo probably mean it's a 3 speed, Under/direct/over gears which allows you to split each main transmission gear into three different ratios. Fun stuff! The first big rig I learned to drive was a 1958 B-73 Mack with an 855 cubic inch 262 Cummins Diesel engine, a 5 speed main box and a 3 speed auxiliary- - - -15 forward speeds and two shift sticks to play with!
Jerry
