The reason the Butler Sidelights were used was lighting regulations required lights to be a maximum distance in from the extremities and the Jeep marker lights were too far in, and were shielded.Regulations stated that the light should be of an area not less than 1 square inch
Butlers were used on a large number of British Vehicles - Bedfords, Austins, etc as well as pre-war civilian vehicles of all makes.
The masks varied and I have had several different types. Some reduce the hole in the centre to about 1"; others had raised rims - for what I have never discovered, except I have only seen them on Fire Engines (!),So: they were already in production, were easy to fit , and the Army had large stocks. I was still able to buy them sealed in boxes from the Vintage Suppliers quite recently. Production carried on after the war - the early(1948-51) Series 1 Land Rovers had them recessed into the panel below the screen, and had 5" Butler headlights in the grill; Tractors of this period had them too.
Also you have to remember that Britain was staggering under a huge debt from the War, and it was a period of austerity like never before, so we used what was available
I recall back in 1983 there were French Hotchkiss Jeeps(WOF) being released and a few years earlier the Greek Army was disposing of their stocks, and several bodies found their way into Britain via individuals and dealers/collectors, but there were no stocks left in the British army much after 1962.
I don't know of any one using Butler sidelights in Europe as there were lighting manufacturers in their home countries catering for their home markets - but it is possible, of course, that surplus supplies were used until they ran out.
I hope this casts some light on the matter
Exeter Devon UK
FORD GPW 202055 DoD 5 - 22 - 44 Serial# 20534360
MBT # 25553 DoD 4 - 43
MATCHLESS 1944 G3L C5806205 (Ex 33 ZC 80)
Piper Cub 1944 L4H 480015 G-AKIB
1942 CHEVROLET G506 CARGO. #2NM 213219