Yes, you are exactly correct. The visible head part of the aluminum "bolt" is 1-1/2" diameter and the shank is 3/4" with 3/4 external threads on its end. It is internal threaded 3/8-16 to receive the eyebolt. The strongest installation is into a cross sill retained with a hex nut inside the cross sill. The cleanest appearance is to countersink the head about 1/8" into the wood similar to the offset washers. That way you can remove the eyebolt if it is in the way of something you want to lay on the bed floor.
These were a little expensive and I believe never made it into the paper catalog. It might be worth checking with some Mar-K customers like Classic Parts or LMC to see if they still have some in stock. I have been away from the business for many years so really have no idea of the status of the parts.
If you have access to a lathe and 1-1/2 aluminum bar stock these are not difficult parts to make. I have never seen a similar product from another supplier.
Good luck,
Mark
