I've seen them, but not in "quality" instruments. It was apparently a feature of some "student violins" ca. 1940-50s(?). I believe at least two kids in my high school had violins thus equipped in about 1953 - '55. Both were rather quickly replaced¹ by "something better" at the rather strong urging of the orchestra director. I'm not sure that it was the pitchpipe, as they may have been 3/4 size and the kids had just outgrown them by the time they started HS.

¹ As I wasn't in the orchestra the main reason I saw them was that they became the "seconds" that the kids brought to camp meetings. My impression was that the kids were among those who played 'cause mommy insisted, rather than being among those who were "gifted," or even very inteested, who would be expected to have been given a "good" instrument from the start.

It sounds like an "accessory" that might have been "addable" to any instrument. It could be difficult to tell, but close examination might give an indication of whether there had been a "rework."(?) It might affect the best search tags if it was an accessory rather than a builder's standard feature.

John