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I would guess that as long as the slug you want to modify is at the same frequency that you want it to operate at, then adjusting the value of the internal resistor may be all that's needed to scale it.
I guess you could also scale the slug power without opening up the slug, and doing it in the meter instead, but this would scale the power for every slug you put into it then, which doesn't seem like a good idea. Scaling is just a matter of voltage drop.
Well, RF has always interested me and I've finally acquired some decent instrumentation and tooling along with assorted components, attenuators, couplers etc.
Refitting old gear with new tricks is also an interest of mine and this seems like really vintage stuff.
I plan to leave the slugs intact and remap the scaling for the meter , prob in a 1/4, 1/2,1,2,4 stepping. Amplifying millivolts (42mV FS), passing that into an ADC doing the log mapping and then reversing the process to drive the meter will take a bit of doing.