More HO model railroad history

A few days ago I came across an old Mantua HO box car kit, probably from the 1940's, which had an interesting note on the end of the box.  It said "your car sides are not in this box - select them from your dealer - they are included in the kit price."  The other end of the box proclaimed it was an all metal kit.  Presumably the car sides were also metal.  I do not have that kit nor the car sides, but I thought that was an interesting marketing practice way back then.  The kit price was $3.50.

What I am very familiar with is the early printed card stock sides, particularly the colorful billboard reefer sides and I still have a stock of them saved for I do not know what.

I used to collect old HO kits but have since disposed of almost all of them as I pare my model collection down to what I actually intend to use.