We used a similar setup in the Navy to check center frequency on TACAN flightline test sets and 243MHz Emergency beacons. Both were swept frequency output, making a freq counter unusable, so we heterodyned/down converted the output with another frequency (offset by 10-60Mhz). Both went into a simple diode mixer, and the difference was either displayed on a scope, spectrum analyzer, or fed to a frequency selective voltmeter.
Very common usage back in the 50s-70s, before GHz-capable spectrum analyzers, scopes with 500MHz (+) bandwidth, and DSP filtering.
Cool to see the setup for TV IF Sweep!
Someday I'll get the gumption to do your VA62 mod, haven't the need for it yet.