I recently purchased a trumpet from eBay that I can find no information on. It was sold under the name of Holton, which is clearly machine scribed on the bell. On that bell there is also machined the nomenclature 'B3'. Following the B3 it appears that someone hand scratched two zeroes in an attempt to make it read B300. Why? The features of the horn look to be a LeBlanc...whom I understand were made by Courtois. The following is the site info from my purchase...I hope that with your knowledge and insight you may be able to tell me your impressions of the trumpet. Thank you.
That Holton was made in Elkhorn, Wisconsin, in the Holton factory, about 1966. That B300 is not a Holton designation and was added later. I found someone else who had a horn with B300 added but got no information about what it meant.
As for what model it is, I don't know what to tell you. You would need to find a Holton catalog from that era.
Here's a bit of speculation. Like a lot of US manufacturers, Holton supplied instruments to the military. It might well be a designation added later... but there is no way of knowing, really.
Because of the era and that it's a Holton, it's an instrument that will play well, though of course only you know if it suits your playing style. Good luck with your new horn and I hope it works out for you.