Original light switch bodys was made up of a early bakelite where the bulk (or fill) material was cotton lump. That lump expands with moisture and crack the bakelite, so they don't age well and are brittle at best. So original light switch bodys are rare and really belong on the fireplace mantle or in a museum. The switch knob is very brittle as well, it deteriorate with age as it is made of pot metal. Common metals in pot metal include zinc, lead, copper, tin, magnesium, aluminium, iron, and cadmium. Depending on the exact metals "thrown into the pot," pot metal become unstable over time, as it has a tendency to bend, distort, crack, shatter, and pit with age.

I am not saying an original switch is not usable if you find one, just be very careful and take good care of it. 

Inside the switch body, the brass buttons of the switch mechanism can leave a trail of brass residue, grease and dirt that acts like a current leader. The switch is easy to take apart to fix that leakage with a little bit of cleaning. On the same time a layer of clear coat or lacquer can be applied inside and out to seal up the body and prevent further degradation from moisture.

Led lights can be very sensible of current interference so just that the leads are twisted can make leds glow faintly.

Philip Jenner wrote:

My original switch  although in nice looking condition    (new lever) added 

has a small leakage ,Enough to make the LED tail light  glow ,My other  101 has a Reno CZ    reproduction   and works fine

regards Phil