No Crank Issue

I’ve been trying to wrap up my project car, a 1974 Dart Sport. Cloned it into a 1971 Duster.
As of a couple days ago, it cranked and started. Yesterday, I was welding in tail light brackets and forgot to disconnect the battery. Now it won’t crank. I can jump the relay, and with the key on, it starts. Previously I did the Mad Electric ammeter bypass. The fusible links to and from the relay have continuity.
This car has that damned seatbelt interlock. It has the white plastic box up under the dash, near the fuse block. However, it does not have the gizmo under the hood that has the bypass button on it. It does have what I show here in the picture. I’ve never seen this set up before, so I don’t know what this is. The gizmo in the picture clicks when I turn the key to start.
Did I fry the interlock? How do I find out? If I shoot the car, will I feel better?
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