Since I had my fork in hand...comparisons with @just jacques pictures show that Fournales made both ISO and Pre-ISO versions of the adapter over the life of the forks.
My Pre-ISO adapter has been cut by someone to work better, but it is still possible to see the differences, and identify which version you have:
Thanks a lot @danson67, very helpful. There is very little documentation that I found on the internet and none of them seemed this explanatory. The only thing I found useful without any measurement taken is looking at the adapter from the bottom. The pre-ISO has just a square guide ( all four sides are the same) while on the ISO version one side has an extra hole (for a bolt???). But again your pictures are really handy.
Another one of my concern,
What is the realistic value of such pre-ISO adapter and would it be suitable for riders on the heavy side or makes more sense to CNC machine it after measurements taken?
Thanks a lot @danson67, very helpful. There is very little documentation that I found on the internet and none of them seemed this explanatory. The only thing I found useful without any measurement taken is looking at the adapter from the bottom. The pre-ISO has just a square guide ( all four sides are the same) while on the ISO version one side has an extra hole (for a bolt???). But again your pictures are really handy.
Another one of my concern,
What is the realistic value of such pre-ISO adapter and would it be suitable for riders on the heavy side or makes more sense to CNC machine it after measurements taken?
Well, Pre-ISO calipers and hubs are pretty rare these days, but don't cost much if you do find them (since they're obsolete)...usually Formula or Magura. I've seen the Fournales adapters sell for £50+, but sellers are often not clear on the ISO/non-ISO difference, so buyers might be going in a little blind.
I'm 75Kg, not aggressive XC rider, and made some adjustments to another of my Pre-ISO adapters and used it for several years without problem.
At the time I only had files. These days I'd use a milling machine and do a cleaner, less invasive job.
Ideally you use a caliper that is direct to ISO (bottom), rather than a more modern one that requires the additional adapted to Post-mount (top):
You might be able to thin the threaded mounts on an ISO caliper itself to glean some thickness...or find a 1998/9 Magura Louise with matched hubs, which I think were pre-ISO:
I had ISO Hope Mono Minis and standard ISO hubs, so I thinned the faces of the brake tab where the caliper contacts. They ended up at around 3.5mm thick, rather than the original 6.7mm. Obviously not ideal, but they worked for me.
Also filed a bigger chamfer into the hub side to clear the button-head rotor bolts.
This isn't a particularly risky area to remove material, but ultra-low head M5 bolts are available which would reduce the rerquirement (1.5mm high flat head rather than 2.75mm buttonhead).
And notched the area between the bolts to clear the piston body of the Hope Mono Mini caliper:
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