I have a 158.1050 with only a zigzag plate. But I have a full-sized 158.1946 with both plates. They are the same size. So keep looking at attachments on Ebay, for all 158 series machines.
I went and checked my machines, both manuals give 38295 for the zz plate and 38296 for the ss plate. I have a .pdf copy of the replacement manual that covers the 1020-1050 machines and it lists the same part numbers. so I would guess these numbers would *probably* fit the 1040 as well. Here is on on the bay:
http://www.ebay.com/itm/NEW-KENMORE-...item43b44bf4d6