Can't supply you with a complete list of makers- but any business dealing with piano tuners carried them.. Tool company Richard Timmins & Sons of Birmingham showed them in their catalog of 1845, selling for 18 shillings the dozen and in all the natural pitches - A,B,C,D,E,F, and G. They reused the plates in their catalog enough to make it possible they were selling them earlier, perhaps circa 1820: but they were also selling threaded Pianoforte tuning pins on the same page, so perhaps not too far before 1845.

Birmingham had become noted for small metal objects in the 18th c., producing buttons, bells, door locks, flintlocks, needles, nails, and what was generally called "toys", small useful metal objects like corkscrews, pipe-tampers; and tuning forks would be called toys. It was his business manufacturing toys and similar items that paid Matthew Boulton enough to be able to finance his steam engine ventures with James Watt. But though the shop must have been in Birmingham, I have no idea who supplied Timmins & Sons with their tuning forks.

You can also find them in hardware supplier Hammacher Schlemmer's catalogs for Piano Materials , circa 1900, along with three different grades of steel music wire. Similarly, piano tech suppliers like Tuner's Supply, John Schadler & Sons, American Piano Supply and Schaff Piano Supply all carried them. But who actually made them? Good question. Schaff still exists: you could call them up and ask them.

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Philip Walker, (ed): The Victorian Catalogue of Tools for Trades and Crafts ( Studio Editions, 1994)

Hammacher & Schlemmer Catalogue No 275: Piano Materials

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