Sure enough: They're reach plates
On page 21 of the February 2007 issue of Farm Collector, you picture two unknown items, and suggest that they might be inspection covers from a corn shredder or toolbox lids. They are neither.
The pictured items are from the frame of a horse-drawn wagon, and are called "reach plates." They lay horizontally in the frame, with the coupling pole sliding in between them, similar to the two slices of bread in a sandwich. By sliding the coupling pole forward or backward, you adjust the distance between the front and rear axles, determining the length of the wagon frame. One end of each plate has three holes across it. A pin would pass through the center hole and a hole in the coupling pole. By searching for "reach plates" on eBay you may see similar items from other manufacturers. The logos may be different, but it is readily apparent that they are for the same purpose.
- Tom Gipson