Joined: Jan 2010
Posts: 9,813
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
Again, and not meaning to state the intentions of any frame's builder, but any JIS fork or frame can be quickly cut to a very-precise ISO dimension.
The ISO standard as applied to cheaper bikes was problematic in that there would not be any "leftover" metal to machine to ANY standard.
A single exception is the Alloy fork, as on a Specialized Epic, which had a bonded sleeve junction with a reduced steerer tube. The alloy part of this sleeve joint interferes with the most popular crown race seat cutters, so the fork crown race seat must either be filed or lathe-turned (I've seen it done both ways).