A metal ring shaped like the letter D. Attached to the floor of a trailer to use for strapping or chaining down cargo so that they will not move.
While then can be attached to plywood using screws, it is best for them to be welded to metal for greater strength.
This must be done during the construction of the frame. Make sure you specify this when you place your order.
There are some D-Rings made that can me installed after construction that have u-bolts that go through the plywood floor and wrap around a crossmember. These are much better than the ones that simply screw into the plywood floor.