This is a Berg Larsen 100 1M offset mouthpiece for tenor saxophone. Based on the shape of the table's milling marks, it's from the "Denim" era. It's older than the ones with horizontal stripes. Also, although the opening is marked as 100, I recall that the music store where I bought it measured it as 105. (This seems to be a common occurrence with Larsen mouthpieces) The 1M is great for producing Larsen's characteristic mellow tone, as well as a punchy sound.