Pull one of the plastic plugs and you should see a rectangular slot in a steel bracket. Its sort of like a skeleton key hole. The corresponding rotating  “pins” which are on the four corner arms of the picture frame drop in those holes and rotate 90 degrees so that they cant lift out of the hole. If it has the plastic caps id think it has to have the keyhole brackets. My plastic caps are rectangular on the underside  and shove down into the keyhole   If you look under the bed you should see the brackets in the area of the plugs in the bed.