I have not seen or heard tell of a 6" fixed sight .357.
Doesn't say there aren't any out there, but they can't be common.
The M520 was a single spec gun, supposedly made for NYSP but not delivered.
The S&W model 65 , 357 magnum , came in 3", 4" and 6" barrel length's and had fixed sights. The 3" and 4" were more popular but there were a few old school officers who always preferred the longer 6 " barrel.
Google S&W model 65 and check it out.
Gary