That’s probably a Merit aperture screwed into the Lyman. They’re still available if you want some more. Very nice EG, I have it’s twin that is one of my favorite rifles.
Merit for sure . I have them on all of my receiver sights. I have used them since the mid 60's. They were out of production for several years but they are back now. EABCO stocks them for the Williams sights thread but it also fits most Redfield and Lyman sights also.https://www.meritcorporation.com/