This was a hard find. I believe I am the first to correctly identify these. If you look at the typical A Clockwork Orange costume, people basically look like idiots. They find their dad's khaki's and some cheap, white suspenders and even ::gasp:: a belt. Now Pete's suspenders, for one, aren't white. They are more of a taupe or sand color, and they match his codpiece. They also don't clip to his pants - they have leather trouser savers (let me know what these things are called) that button to the inside of his pants.
Now look at the back - they look nothing like normal suspenders or even Y-suspenders (which are what Alex wears).
After scouring the innernette, I have determined what these suspenders are: British WWII trouser braces. These were worn with battle-dress trousers by all Commonwealth troops (British, Canadian, Australian, Scottish, etc). They are stamped with the year of manufacturing, and the suspenders I have are dated 1945. Good luck on your hunt!