That meter has been made with more than one circuit board in it. The trimpots that serve to set each scale's calibration are marked clearly in some versions.
Not so clearly in other versions.
We use Bird meters that have had the elements checked for accuracy. The meter being calibrated goes in line with the dummy load, and the trimpots set to match the Bird's reading.
If the trimpots are labeled "20w", "200w" etc, it's not a big deal.
And if they're labeled "R4", "R5" and such you'll have to trace each wire from the range selector to the trimpots to identify what each one does.
Here is the only manual I have for those. Pretty sure the circuit board in this one is bolted to two screws against the back panel, with the main wire from the "input" coax socket passing through the center of a doughnut-shaped toroid coil on that pc board.
Don't ever remember seeing a diagram for any other version.
We learned the hard way to make sure that the coax sockets are properly grounded to each other. Some versions simply rivet the coax sockets to the painted rear panel. Doesn't penetrate the paint to the metal back panel. I have some pics of what we do to bond the ground side of the coax sockets to each other and to the pc board. I'll upload them to post here when I get the chance.