What's a transmatch? It's a device for matching a transmitter (or a receiver) to an antenna, or perhaps the other way around. It's an impedance-transforming network. They're pretty common in amateur radio and you can even find high-power versions at some MF AM and SW stations.
This one was made by the perfectionist James Millen Corporation, their "Junior" model (manual for 92201 here), rated at 150W continuous, 300W peak:
Internal construction is as clever as most Millen devices. The split-stator input variable condenser sits on top of a little copper-plated box and the input connector is at the rear-wall end of the box. What's in the box? A standing-wave bridge, a pair of directional couplers for providing equal-amplitude samples of forward and reflected power. They go to a front-panel selector switch. Set the front-panel meter to full-scale in "Forward," flip it to "Reverse" and adjust the tuning controls for the lowest reading. Voila, matched!
The output capacitor stands on ceramic pillars: both sides are at an RF potential above ground.
On the back panel, an extra connector! It connects to a one-turn loop coupled to the main coil (see the manual), and provides a sample of the signal for an oscilloscope or keying/modulation monitor.
Found on an auction site for a remarkably low price, this example is unrestored. Typical of Millen products, it is overbuilt and will only need attention if the meter movement or passive components in the SWR bridge (two diodes, two resistors, a fixed condenser or two) are damaged.