In what set is this brick used? In which set is this LEGO brick used? There is no number in it and it's original LEGO. The only number on it is "A 1".
The specific part (870) is identified in the other answer, however it is part of an assembly with its own part number:

https://www.bricklink.com/v2/catalog/catalogitem.page?P=827#T=C&amp;C=3

827 Vehicle, Tractor Chassis

870's page mentions it has extra parts that it is used with, and 827 is the complete assembly (steering wheel + mechanism).

This page details what other parts are needed to complete it.

https://www.bricklink.com/catalogItemInv.asp?P=827

870 is the base in the picture, and 871-874 are the mechanism pieces.