This was their large version, with sizes smaller and larger than what is used in modern pianos.
The smaller Starrett gauge, No. 280, is the one that was most used by piano tuners. The Starrett No. 280 was a direct copy of the Darling, Brown, and Sharpe gauge pictured below:
L.S. Starrett also made an action center pin, and tuning pin gauge. These were not big sellers; tuners would use a micrometer for center pins, and later on, a dedicated tuning pin gauge capable of measuring tuning pins in the piano.
“Should measure,” are the operative words here. There are actually significant differences between the wire sizes of various nineteenth century string makers, despite the fact that they used the same gauge numbers, which present- day restorers of historical and elder pianos are well aware of.
This is a chart of wire sizes as measured by Puresound, makers of stainless steel wire for early 19th century pianos. I figure that it could be considered a snapshot in time, as there was likely variation between batches of string within a string maker’s output. I added the sizes for American Steel and Wire.
Two earlier types of wire gauges as shown by Sievers in 1868. A slip style and an adjustable gauge. These types had advantages over the slot and hole types in that they showed increments between indicated sizes. The slip style is still used for measuring universal bass strings.
Firminy music wire was used by many French piano manufacturers, such as Erard, Pleyel, Gaveau, and the smaller makers, during the late 19th century, into the 20th century.