You are buying a 5/8" x 2 1/2" solid brass hex headed set screw
They are new/old vintage stock with more available

(€1 coin for display only and not included)

We are the 4th generation and have just retired from a D.I.Y. shop that has been trading for over 110 years
as you can imagine we have lots of new/old stock  
With hundreds of screws, engineering dies, brass fixings
and so much more. Please contact us to find out more