Sent a note through the eBay message system to the seller. He responded quickly which is usually a good sign. Here's what he said:
Good Morning Reid
How are you? Hope all is well. I have owned the tag about 6-8 years, was purchased with a group of 100 Healey parts including 6040X carb bodies I needed for my "M" from "Racer Bud" in CA.(See Photo Plastic bag in ad) I know the #1 car came from CA many years ago so I surmise the tag was removed when the engine was rebuilt years ago as it would not survive the hot tank.
I matched the tag to the #1 car when I was looking through the parts book,(See Photo)
I will answer any questions I can, please just ask.
Phil
(The photos he refers to are the ones on the auction webpage; he didn't send me any photos.)
In any case, what Phil wrote checks out very well. Presumably the engine was rebuilt before the current owner acquired it, and so the engine number plate could have been removed before the current owner bought it, and then separated from the car for some reason; it's an easy thing to forget when an engine comes back from the hot tank, and since the plate is pretty beat-up, most probably would not have wanted to reattach it to the fresh engine anyway. If you didn't know (or care) that it was the very first production 100, I can understand how it would not have been a big deal.
I'll email the owner of the car and provide a link to the auction in case he'd be interested in acquiring the item.