It's an "elapsed mileage odometer".
That's what Bentley calls it anyway. Some refer to it as an "EGR counter", others call it a "speedometer recorder". We would have accepted all of these as a correct answer. EGR counter may be the most accurate as it was part of the early EGR systems in the mid '70's and into the '80's.
These things were on many different VWs. They were connected to the speedometer cable and all they really did was count miles. When they got to a certain number, say 15,000, they would trip (like the GFI outlets in your kitchen). This would turn on a light on the dash that was a reminder to have the EGR system serviced, cleaned, checked, etc.
The thing is all you had to do to turn off the light on the dash was press a little button on the back to reset the thing. You're good for another 15,000 miles, (or 30,000 I can't really remember). Sure would have been tempting to just press that button and put off servicing the EGR system until another day. Thanks to all those who participated!
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