Did they calculate it? If so, couldn't have been very accurate.
Yep, spark gap. It's an old method of timing. There was an instrument demonstrated to my physics 101 class in college that used a waxed paper tape pulled through a 60 Hz spark gap to demonstrate measuring gravitational acceleration by the increasing distance between spark holes made in the paper as the tape was pulled through the gap. We had lots of antique lab gear, I recall.