It was at the top of a pole, I think? What is it? Lightning arrestor? Metering device? Airplane engine!?!

Thanks to Mid and others for the answer, it's a primary metering device - here's the details from Joe Maurath:

"This arrangement is for stepping down primary voltage (such as 13,800V) to 120 Volts to an electric meter that would be physically lower on the pole or immediately nearby. Read on....

This is known as "primary metering" for large customers.

An electric meter for such large commercial and/or industrial applications (such as this would be used for) cannot practically have high voltage (viz, 13,800 volts) wired through it.

So, a step-down arrangement is used by electric utilities and this a fine example of one of them.

The primary circuit voltage and amperes flowing through this gear is proportionally transformed for the electric meter. From there a billing multiplier is calculated....

As a jorneyman meterman I have built many of these set-ups from scratch through the years."