PART NO.  SPL.PAGE  DESCRIPTION                                                           
17H5483   15.N.3    Screw fixing lever (flasher switch 1500)
17H5483   16.N.3    Screw fixing lever (flasher switch 1600)
17H5483   TC.N.3    Screw fixing lever (flasher switch) -(switches, electrical, Twin Cam)
17H5484   15.N.3    Washer for screw (flasher switch lever 1500)
17H5484    Washer -spring -screw -(flasher switch, switches, 1600 and 1600-MK-II)
17H5484   TC.N.3    Washer for screw (spring) -(flasher lever, switches, electrical, Twin
    Cam)

This is the securing screw for the bat handle on the turn Lucas signal switch (with vacuum delay shut-off). It is a Cheese Head screw, number 5BA thread, 1/2 inch long. I believe it should be bright nickle plated (or chrome) for appearance. Some modern replacement parts might be zinc plated (dull when new and fading darker with age).

Some modern replacement switches may have metric threads and metric screws, and those will definitely not be interchangeable. If you buy a new switch, the original screw may not fit, and if you are going to buy a new screw, you need to know if you need the original 5BA screw or a modern metric screw, definitely two different part numbers.

you will not find the British Association screw at your local hardware store, at least not in the USA. However, BA screws are common with instrument makers and model trains, so maybe look for a hobby shop selling/servicing model trains.

BA thread standards re well known, so skip that. Just so you don 't have to ask or go searching, I am posting the dimensions for the screw head.

5BA screw dimensions in inches:
d  max      0.1260
dk max/min  0.2210/0.2110
k  max/min  0.0880/0.0830
n  max/min  0.0400/0.0340
t  nom      0.0400

#5 Medium Spring Lockwasher 
dimensions in inches:
A  max/min  0.133/0.127
B  max         0.236
T  nom         0.031