I've actually started using calipers to measure round and roundish things, and surely someone else could use as pair as well. I like these, probably made by a machinist late 19th c/early 20th c or so, especially for the fact that they are monogrammed with three initials on one side (FVZ, which suggests an interesting name1) and four on the other--the latter perhaps the abbreviation for a business, or perhaps a High School (HS)?  Substantial in size and weight and really nice in the hand.

5 3/16" long, opens to about 5 1/2" wide, very good condition.