Genuine GM NOS as shown. Group 4.661, Part #5300863.

This is a "Master Cylinder Main Connector Bolt", it fits the following GM Cars:

1936 & 1937 Chevrolet Except GA, GB Models.

1935 thru 1948 Pontiac All Models.

1934 Thru 1950 Oldsmobile all Models.

The parts books say it is "1/2-20x17/32" thread - 1 13/32 O.L., Tapped Hole in Head".

It looks like this is the connector Bolt that fits on the master cylinder that the Hydraulic Line goes into.