Originally Posted by
T-Mar
Actually, I believe the inscribed groove indicates an English thread standard, which makes sense when you think about it. Campagnolo is Italian and most of their sales are to Italian manufacturers who would specify Italian threads. Adding the groove is extra cost and they would want to use it to designate a thread standard with a lower volume of sales, to mitigate the impact of the extra manufacturing operation on the profit.
Regardless, as stated, English and Italian freewheel threading is compatible with only minor damage resulting to the threads. Unless the OP is a very strong cyclist or changes back and forth between English and Italian threaded freewheels, there should be no issue.
The groove is indeed English. This gets confused as Campagnolo did not always do this, and it was before they started stamping the thread pitch, and English and Italian are close enough that many did not know... the difference is that while the pitch is the same essentially, the inclusive angle of the thread is different, one is 60° the other is 55°, and I forget which is which. If I remember English on Italian is a bit loose, and Italian on English knocks off a wee bit of material, so Italians get 55 °.