Slots on each side of the boards sounds like twice as much labor for Henry as necessary. That doesn't sound like Henry's style.
During manufacturing, would they have had such a problem with bad wood that it would have justified the extra effort to put slots on both sides of the wood? Or would they have just tossed the occasion bad wood board, since material was cheaper than labor?
On trucks, Henry didn't even care if the sheet metal paint wasn't perfect, so why would he care if a board had an ugly knot, especially since the knot was painted over anyway? If the knot was structural, they would have tossed the board.
Lastly, the square washers take a good 'bite' into the wood, regardless of the presence of grooves or not.
Or, all my assumptions could be completely wrong
