I have three King M21 mouthpieces.

One is a 12ish piece with no decorative (coin edge) knurling. Bore = 0.227

Another is a 7ish piece with knurling at the rim and at 1 3/4" from the stem end. Bore = 0.227

All pretty standard stuff.

The third mouthpiece is a mystery to me. It has a 12ish rim, a bore of 0.232, and two rows of knurling, with the lower knurling only 1 1/2" from the stem end. I can't find any reference to a King M21 with a 0.232 bore. Also, I thought that only the 7ish M21 had knurling, and this one is a 12ish. All the silver factory plating is still intact, including in the throat, so it hasn't been re-bored. It is marked with only the standard King logo.

This piece sure looks factory original. Any ideas about its history ?

I’ve got a King 28, 12C, M21, M31, and 11M. There’s a #24 out there. I’d have to grab some calipers to get specific, but I’ve had a very hard time finding any true specs for King mouthpieces. Chris Charvat at the H. N. White store on eBay (hnwhitecompany) may be the only person I know who might be qualified enough to answer this. I don’t think they really sell mouthpieces, but they keep a stock of them to sell with the instruments. They do however have a lot of the original specs as they’ve kept everything in the family. If he has the time to answer you, he might know.

I'll check that out.

Thanks for the lead.

I have one King M21 mouthpiece. It came with a 1946 2b that I'm the second owner of and is in really good shape. It matches to your second description "7ish piece with knurling at the rim and at 1 3/4" from the stem end. Bore = 0.227". Perhaps that will help you date yours?

The variance in mouthpieces of the same brand/size bothers me, but apparently many brands have either been sloppy with quality control or changed their own specs from time to time.

If you can find out details about series and dimensions of King mouthpieces, CharlieB, please post it since I am very interested as well.

Thanks.