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.