Self-made antenna coupler HF band

I acquired a rig for mobile operation and purchased this item for it, but it turned out to be surprisingly large.

The manual from the manufacturer is included.

Comments from the seller at the time of purchase:

This was self-made using a coil purchased at the Mizuho Tsushin booth at Ham Fair. I was satisfied with making it and haven't used it. Therefore, I have not confirmed its functionality. It uses a variable capacitor that is difficult to find nowadays, so how about using it as a base for modification?

It's difficult to explain the function of the knobs in a single word. Since I haven't actually used it, I don't know the order in which to turn them for smooth adjustment. It is for connecting in the order of transmitter - VSWR meter - antenna tuner - antenna, and turning each knob to bring the VSWR as seen from the transmitter to 1.0. I believe it is necessary to understand the principles through the internet or books. I plan to include copies of reference materials.

For connecting the connectors, the rear left is for the transmitter and the rear right is for the antenna.