Seat support vertical pieces
There's a pulley system in the Jenny control system that operated the ailerons. This 3 3/4 in long block was made to accomodate the forward pulley. It is here where the aileron cables make a turn and exit the fuselage.
Now, the fun part. The rear vertical supports must attach (at the bottom) to the floorboard supports and the top bolts to the rail. No real issues here.
But, the forward verticals attach to the floorboard supports also at the bottom, but the top is somewhat different. I have seen two ways of making this vertical. The first way I saw on two different Jennies. The vertical piece was cut flush with the bottom of the rail. Then a long bolt that holds the pulley I mentioned earlier goes through the rail and holds one side of the vertical in place with an l bracket. There is nothing holding the other side. You can see that below in the Golden Age Air Museum's Jenny.
The other method was seen on a Jenny displayed at the Naval Aviation Museum in Pensacola FL (picture below). The location of the vertical was not critical of the pulley and mirrored the rear vertical in shape. It was held in place by a bolt through the rail.