$600 for a 4:1 thingy is ridiculous. Someone made a run of repro 4:1 units that were resonably priced (reasonable is a relative term, of course) not too long ago. Under $200, as I recall.
For that amount of money I could learn to tune very s-l-o-o-o-w-ly without the 4:1 :-)
Now that that sale on e-pay has occurred, everyone will want at least $600 for a NIB spinner knob and vernier mechanism. Used units will go for less, but if in good shape, not much less.
Considering the current, going price of the receiver, $200 is not bad. That's what a repro 6-kc AM mechanical filter, that works identically to the original, is sold for in the Electric Radio ad.
Lacking a stock or repro vernier mechanism, a substitute could be fabricated using a National Velvet Vernier dial, similar to the ones on the BC-375 tuning unit. They were sold by National as a type "AM" dial. (I would avoid dismantling an original pre-WW2 type A round bakelite dial - they are too valuable).
Without drilling any holes in the A4 cabinet, a bracket could be fabricated to mount, using the two tapped screw holes on either side of the shaft hole, originally used to mount the stock 1:1 feedthrough bushing. The replacement $600 vernier mechanism originally made by Collins employs those holes for mounting purposes. The 1-100 dial skirt should be discarded since it serves no useful purpose in this application. A bigger knob, to cover up the outer edge of the vernier mechanism could be used if one is found. I have never tried it, but the tiny dial skirt used on the BC-375 T/U could be modified by enlarging the hole in the middle, and the rim epoxied to the tuning knob - epoxy sticks well to both metal and bakelite, so the two ought to be able to be easily fused.
This mod wouldn't look as nice as a stock Collins vernier, but it would certainly be less ridiculous than the steering wheel shown in the photo.