In wooden railroad car construction, there are castings, used to trap the head of bolts (usually square head bolts) which pass through the floor of a car to prevent them from turning when being tightened. In this case they are the bolts which hold the platform timbers to the sills.

Voss includes drawing of 4 examples. Some are square and are mortised into the sills or floor, some are round with "wings" which are captured by the floor boards.

I made a pattern for a SP part for a boxcar restoration some years ago, but that was for 2” thick floor boards… my current caboose project has much thinner boards, so those castings won’t work.

I can fake up something with wieldments which will work, and be hidden under the finish floor. I could make new patterns for cast parts, but the design would be a guess, and it would take time, too much time for this high speed project.

One of my first quests at the IRM years ago was to replace a rusted off 5/8 square head bolt securing a grab iron on our PFE reefer. The head was buried between the sheathing and the ice bunker. What a job, took me three days to get it out.
It was secured to the inside of the diagonal sheathing with a rather heavy gauge rectangular piece of sheet metal. About 2” by 6” with a hole about 1.5” in from one end to put the bolt through to the outside. The long end was folded over the square head, and then nailed down to keep it from backing out or spindling. It also took the place of a washer to keep it from pulling through the wood when torque down.