A much sought-after instrument in the antique market is the 1740 vintage caliper (British Patent #490) that was used in the Royal Navy for two centuries.
Now made by an instrument maker in India, it comes in a presentation box with a classic fouled-anchor brass inlay in the lid.
The adjustable friction joint makes it ideal for single-handed adjustment and use.
You can use it one-handed, putting pressure on the arches to open it or pressure on the legs to close it, keeping the other hand "free for the ship".
It makes a great gift for latter-day admirals. Just under 8" overall.