What the heck is this?

[regguy1]

Found this among tools from a machinist, I'm not able to determine what it's for. Knurled rod on right is fixed / rod on left rotates and causes center wheel to rotate.

 

[KnurledNut]

Machinist speed indicator.

 

[sgrammel]

Pardon Our Interruption...

What he said ^

 

[RTM]

Correctly, it’s a revolution counter, that you pair with a watch, and it gives you RPM, but that’s just being pedantic.

 

[seber]

That is what I still use. More reliable than all that modern nonsense.