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Rick, I was told it might have been some sort of alarm system or something?

Kind of doubt it though considering the car was stolen shortly afterwards from that same spot.
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That looks like an alarm horn to me - Look at the on off switch mounted on the fender. This was the way it done
back then!
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My '66 had the alarm switch in between the flags
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The alarms were easily over come - they were mounted the way the one in the Chevelle was - facing down.
A thief would just crawl under the car and jam a screw driver into the alarm siren and disable it!