This is a Bob Reeves brass trumpet mouthpiece, model "MS39S". The general smallest rim size for Bob Reeves mouthpieces in Japan is 40 (Bach 10 1/2 to 12), but this is the MSS model (supervised by Mr. Tadashi Sugiyama) which uses the slightly smaller 39 rim. The MS model uses a somewhat deep V-shaped cup, but this MSS model uses a shallower V-cup, making it easier to get used to for those who seek a clear brightness and edge in their tone. I believe this is a size that is not very common. If you were looking for one, how about it?