7m18 Lure with Mirror "spot" or defect?

[Insidiator]

I inherited this lure a long time ago, changed the hooks and put in in the "do not use" section of the tackle box. Recent trip to Fla, nothing was working at all so I pulled it out and twitched it through the clear water there to observe the action. Looks like the "spot" occasionally threw a bright flash which may have been designed in as a feature, or is it a defect? Or was it altered? I haven't found anything like it with a spot on Mirrorlure's website. Anyway, third cast caught a 19 3/4 Spec, and later I caught a second just like it. Just wondering if anyone knew about this "spot", it's starting to bug me.