looks like a bit from a milling machine

This is an industrial "dado" blade which is used for cutting grooves the shape of the blades into wood, normally used for joining. Grooves that shelves fit into or something like that. The notch is called a 'key seat'. A rectangular piece of metal would be placed in a matching groove on the shaft to insure it turns at the same speed as ther shaft and won't slip. I have the same type of thing for my power saws at home, but this is much more 'heavy duty'. It's not for metal, or the blades would be carbide.

It's called a slitting saw and it is for metal. It is used in a milling machine to cut slots or grooves. It would not work well at all in wood because the clearance angles are not big enough (it would burn the wood) BTW, I am a machinist by trade. Tom