Yup, not a 21 from the looks of it. Got home to my computer with a large screen to take another look. From my sight book, it looks like it might be a Lyman 34. They were made for the Springfield 1903, 1906; Krag 1896, 1898; Lee Enfield Mil and Sporter; and the Remington Lee. Should be an application code underneath the sight, a one or two letter code. Have you had it off to look underneath?
Matt