The rockers in old Enfields use reduced 1/8W nuts to hold them down.
I've found that many makers (Douglas comes to mind) avoided the 1/8W hexagon, and instead used 3/8AF. Who knows what that was all about.
I have a Lennox motorcycle made in Lennox St, Richmond, Melbourne, c1915. The engine was made on site, and I reckon they also made most of the nuts and bolts: the hexagons are all "AF" (I was going to say "Imperial" - certainly not Whitworth) but most of the threads are BSW!!! I suppose they were turned from "AF" (1/2, 9/16,...) hex bar, but again who knows. I certainly need the full range of spanners in my shed.
As an aside, when I was doing a deep clean on the Lennox I took most of it apart, cleaned and reassembled. I had problems finding a spanner to fit some of the smaller nuts on the mudguards. On closer inspection, I found that the nuts had 8 sides! The last owner of the bike was a plumber, who used the bike and sidecar for his work during WW2. He used gutter bolts where appropriate, but was clearly offended by the square nuts so cut the corners off to make octagons. I've had people tell me I should get rid of the gutter bolts, but I love that part of the bike's story.
Leon