Wondered about that also, imperial increments, English characters and spelling. The O-ring too, until demonstrating the divisions indeed fell on the scale correctly. Most O-rings are compression or transfer molded. They are very round and consistent round cross section, with reasonable density through out. I'd bet he worked from the plexi disc out, first adding the O-ring for correct circumference with a certain degree of stretch, to 'calibrate'. It's a nice neat project. Wouldn't build a house with it, but checking pipe circumferences, heck yes.
I'm on my 3rd pocket Pi pipe tape, at ~3/16th's wide, awful fragile, compared with replacing a nicked O-ring.