That Hide-a-Key looks pretty new, so likely a later addition, especially with your suspicion of the ignition switch being wired incorrectly.
I would not use that saw blade as patch material. High carbon steel is not what you need. It can cause brittle welds.
That metal tag on the seat springs is the part number. The S G is likely the manufacturer code or some such. My seat springs had a paper tag with the PN on it. Mine said L.A.Y. 3696078
