If you are talking about the bolt head size on the two bolts that hold the pipe up,they probably are scaled down to a smaller size. It happens a lot on exhaust systems. If the pipe doesn't want to drop,there is a holder(hanger)that bolts the pipe to the side of the bell housing with 2 of the clutch pan bolts using the same bolt holes.

I think the only reason they were slotted was for some minor spacing differences. They made them with a vulcanized rubber connection between both metal parts to dampen out clutch chatter. Steele makes complete new ones. They were for anti clutch chatter so both ends were to be tight and the rubber damped out possible chatter.

You should have a very thin chromed metal strip that snaps down over the hood center. All 1937 120 and 115c all had them on originally. If you can't find an original, Max Merrit has repros(though not as nice as originals,miss the slight center crest).

This may help you, if I understood your question about springs on the hinges. They only should be mounted one way,to prevent braking them.

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The 1937 6 & 120C the hole spacing,comes out to approximately 14 inches between the legs center to center.

This may be what you were looking for,as this copy of a installation sheet is for 1937 6 & 120 and some Seniors if you look closely it shows the switch placement and wiring for a heater/defroster in 1937. I would have sent it sooner but have been trying to make an attachment to this work.

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May guess is a wire was run along side of the wiring harness as it returned to the cars interior or dash. As these were a dealer installed items,the wire wasn't part of the wiring harness. If you are installing a new wiring harness,tell the maker that will be using a backup light and any other accessory as Fog or driving lights and they will add it to the wiring harness for you.

The trunk rack was,but you will need longer bumper braces.

As to finding the needed parts,check out Tuck211113 on Ebay no.121564098719 who has a pair of them new. These are his last set.

A good place to start is 1937 GM products heaters as Chev. ,Pont. or Olds. It seems like they always sold Packard last years model year and put on different knobs and emblems and paint. I have a rough 38 Packard std. heater you could have for shipping cost, but the core is of unknown condition. I believe the deluxe heaters may have had a thicker core,but the same outer case with extra case or housing embellishments.