Yesterday I looked through my mouthpiece collection again. I found a mouthpiece called the “Kölner Model”. It's a really small mouthpiece. Inner diameter 23.5 mm, Medium deep cup, throat 4.5 mm: I think it was originally made for a german tenor horn. I put it in my Bach 39. The shank only went 24 mm into the leadpipe but the intonation was ok. The sound is bright and clear but not too sharp with a mellow timbre. Due to the slightly deeper cup, it also sounds good in the lower registers. The sound volume is rather small due to the narrow throat. I think a good mouthpiece for small ensemble play or to accompany a vocal choir. I will test more mouthpieces of this type on my Bach 39 in the future. The german manufacturers Klier, Schmidt and Tilz make several mouthpieces of this size under the name "Tenorhorn"
