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.