What is this nut on a micrometer head and what is the tool for removing it called?

Question

My apologies for a likely very simple question. But what is this nut which I have on a micrometer head called, and what is the tool for removing it called?

Answer 1

Short Answer

The nut on it is called a slotted lock nut or locking bearing nut. You can remove it with a hook spanner.

Long Answer

I guess your micrometer is Japanese and thus probably doesn't conform DIN standards but for slotted lock nuts you can check DIN 981 or DIN 1804 and for hook spanner you can check DIN 1810

Why use Lock Nut

The main reason is screw loosening. With time all screws get loose (no pun intended) and on a device such as micrometer that loose fit will cause errors. That nut, locks bearing into place. Below is a similar Lock Nut for bearings from Misumi USA, you can check datasheet here

Information about lock nuts and hook spanners from SKF but I don't have enough rep to post more links.