The 8021 and 8023 are functionally the same. The 8021 has a recess bored on the front side. Early 23c's had a slide-in filter holder immediately behind the lensmount. It would collide with the rear of similar vintage 50mm f2.8 EL-Nikkor lenses. The 8023, with a recess bored on the back side pushed it just enough out to clear. Either should be fine for most 50-105mm lenses.