The 3 1/2 set from 1935
Here is a rare set that other collectors missed. I bought this set on ebay, January 2002 for $24, and was the only bidder.
The seller had it listed under "Medical./Other Engineering Items" instead of under "Toys/Erector Sets". So if you want to buy something on ebay, don't browse to the category where you think it should be listed. Instead, do a search using the name of the item you want. This will bring up all items, from every category, with that name.
This is a one-year set and contains the architectural panels which are alone worth several times what I paid for the set. It is complete with manual, pop rivets, and paper fasteners; and in excellent condition except the box top which has parts of the aprons missing.