oneclick 

I have one of the ones pictured - the Regolabile - and it's not the one in the photo. It has a triangular bottom section that rides in a V-groove in the quill. It's several bikes deep, but I think the bolt that clamps the bar-clamp to the stem has a cylindrical section that rides in whichever groove is chosen. This arrangement gives only a limited number of discrete adjustment positions, but is less likely to rotate under maximum power (which is a Bad Thing). For the infinitely-adjustable round version it looks as though any stem-clamp-bolt would do, given a pitch and diameter match. You say you have one that matches, almost certainly it is a metric thread, and similarly a normal- or fine- version is probably correct. Measure the diameter over the threaded section, if metric it will be a small amount smaller than a unit mm measure; then you only have two choices.