My friend Anne Finnie blogs over on HP's Next Bench. Recently she did a great post about HP's very 1st product. You can read the full thread here.
Way back in 1939, an HP product was used to help construct the amazing audio in Fantasia. The unassuming -- if a little intimidating-looking -- metal box that you see here is the HP200A, an audio oscillator. That is, it generates one pure tone or frequency at a time. Through the years, HP oscillators were used to design, produce and maintain telephones, stereos, radios and other audio equipment.
In 1938, when a sound engineer from the Walt Disney Studios saw the Model 200A audio oscillator in action, he asked Bill Hewlett to make some modifications to it. The Model 200B was born. Disney ordered eight of the Model 200B at $71.50 each.
Disney engineers used HP 200B oscillators to test the various channels, recording equipment and speaker systems in the 12 specially equipped theaters that showed Fantasia in 1940.
As for why they called the first unit the 200A: To make it seem like the company had been around for a while.