The input circuit that feeds the radio's drive power into the 8417 tube was not designed for low input-side SWR, but to allow you to set the carrier power.
As a result, the input SWR will rise and fall as you turn the small 'carrier' knob on the rear panel.
It was designed to use with base radios that had a tube for the final amp. Tubes tend to be more forgiving about high SWR than transistor finals.
There are two ceramic-disc capacitors that serve to resonate that input coil, one on the coil's input wired across the 47-ohm 2-Watt resistor. The other one is connected from the input coil's secondary winding to ground. The capacitance values used were not always the same from one production batch to the next.
You could 'tweak' the capacitance value of either or both of those disc caps to reduce the input SWR, but this will tend to reduce the effectiveness of the tuning knob as the coil's slug moves in and out of the windings.
The coil serves to reduce the carrier drive to the tube by introducing a mismatch between the radio and the grid of the 8417 tube.
Decades ago I wired a trimmer capacitor in place of each of those two disc capacitors. Found that when I tweaked the trimmer caps for lowest input SWR the knob wouldn't reduce the JB's carrier power very effectively. Diddled with it until I had a compromise between a 'moderate' input SWR and the ability to turn down the carrier. Took the trimmers loose, measured the capacitance and wired in fixed caps close to the values measured on the trimmers.
If the radio has a carrier control, you can use that to keep the JB below 10 Watts carrier output. Odds are you can find a setting on the input-coil knob that gets you down to 2 to 1 input SWR or so.
Probably a better compromise than trying the trimmer-cap 'tweak' method.
Kinda reminds me of a long-selling book called "How to Keep Your Volkswagen Alive. A Guide for the Complete Idiot". Was written around 1970 or so, and detailed how to repair your air-cooled VW, and told you which tools you would need and what to watch out for to do just about any repair you would need.
Except for changing ball joints. Not a word about that. Decades passed, the original author died, and the publisher continued to publish updated editions. Eventually the 12th (or 15th?) edition included a procedure to do this. The book's editor noted they had received numerous requests for it, and finally found a writer to contribute a procedure.
At the end of that section was the question: "There, don't you wish you had spent the same time collecting aluminum cans, turned them in for the cash and hired someone to do this for you?"
For me that answer was "NO". I was glad to have the ball joints replaced and took the opportunity to grease the torsion bars and replace the torsion-arm seals.
But by then I understood what the author meant.
73