It’s a tactile lens-mount alignment marker. On older Pentax cameras, the white plastic bump lines up with the lens release button/lever, letting you mount or remove the lens by feel when you can’t easily see the usual alignment marks. Pentax documentation for cameras like the K1000 specifically notes that in the dark, when the red dots are hard to see, you can align the white plastic bump on the lens barrel with the lens release lever by touch. So it isn’t a light meter, control, or decorative part — it’s there to make lens changes easier without relying on sight.