Picked this up at a muddy March flea market for two dollars. It has numbered notches and a rotating top plate, but no obvious way to attach it to anyt.

That sounds like a classic “mystery flea market tool” — and your clues actually narrow it down quite a bit.

From your description:

…it’s very likely one of these:

🧰 1. Mechanical measuring / indexing tool

Could be an old:

The numbered notches often help set equal spacing or angles.

Some older tools used a rotating dial to:

These often look “incomplete” because they were meant to sit inside a larger frame.

Certain vintage tailoring tools had:

These also sometimes don’t obviously “attach” unless you had the full workstation piece.

A strong possibility is:

it’s a detached component from a bigger device

Examples:

The rotating plate suggests it was meant to adjust settings, not function alone.

If you want a precise ID, tell me:

If you upload a picture, I can usually pinpoint these flea market mysteries pretty accurately.