Yeah, I honestly don't know Jerry. There are 100 bins in it, 5 high x 20 wide, seems a bit much for mail storage in a single household, not worn enough or dirty enough to have come out of a post office. The only other thing I can think of as a possible use, and remain that clean, would be the woman of the house was into knitting, and it may have been used to store skeins of yarn in assorted colors. Just saw a post on Pinterest or someplace on line the other day where the gal had an entire wall full of yarn in similar storage, in a vast array of colors.
At any rate, I thought it would look good full of pen blanks, knife scales, call blanks, stopper blanks, kits of all nature, etc. Could store just about anything in it, if I run a strip of plywood along the bottom of the bins. Nuts, bolts, nails, screws, pipe fittings, small parts of all nature, lay cans of spray paint on it's side. I've got to check size, but I bought, I think 100 of those little plastic bins, on a sale at Northern Tool, that I believe will fit in it. Haven't dug them out yet to check, but if they do it would simply require orienting this thing Vertically. Was already planning something of this nature in the shop when I remodel for the dedicated wood shop.
It could have LOTS of uses, for $40 and a ride to town.
Were I building it, I wouldn't build it that heavy, I would have dadoed slots for masonite dividers to allow more versatility in bin size, oriented vertically and saved a little room in the shop, but there's a day's worth of sawing and sanding saved there, on top of savings on the cost of materials.