I think here under the radar means that locals didn't know the factory was there, not that Zildjian is an unknown brand, it even goes on to say

> Even in Massachusetts many people have no idea an industrial factory outside of Boston designs, casts, blasts, rolls, hammers, buffs and tests at least a million Zildjian cymbals each year.

Right? If you spend time around drummers at all, you almost definitely know the name Zildjian (the same way drummers know the names Fender and Gibson).

"Operated in Norwell" is the verb phrase that "under the radar" is modifying. It's about where it's based.