Type 4 threads are classed as 21.00mm in diameter but will measure 20.85mm with a 0.75mm thread pitch in either 18.4mm, 18.5mm, 18.6mm and 18.7mm bore diameter.
Type 2 threads are classed as 20.50mm in diameter but will measure 20.35mm with a 0.75mm thread pitch in 18.4mm bore diameter.
If your original flush choke tubes are 46.00mm in length the thread will be 21.00mm in diameter and can only be the Type 4 variety in either the 18.4mm or 18.5mm or 18.6mm or 18.7mm bore size.
If your original flush choke tubes are 38.70mm in length the thread will be 20.50mm in diameter and can only be the Type 2 variety in only the 18.4mm bore size.
It might be best to bypass the term Type/Gen 3 unless someone is specifically looking for the flush style choke tubes ?
A vernier gauge or a 0" to 1" micrometer and a thread gauge are usefull to confirm etc.
It's more confusing than unknotting spaghetti ???
P.S. I believe that I am 100% correct in stating that the flush fitting Gen 3 choke tubes were only available in a 38.7mm length and were a Perazzi after thought/alternative for the 18.4mm bore Type 2, 18.4mm bore choke tube treads ?
If you have a 46.00mm "flush" choke tube length "IT'S THE TYPE 4 VARIETY"
Hope the above helps