It's not just China that uses 50Hz more use it than don't, countries that run 110v tend to us it more.
Is the mount for the DS-2CD2335-I made of plastic instead of the aluminum used by the DS-2CD2332-I? If yes, then it is the same as my 4MP DS-2CD3335-I camera. The 4MP camera is a China only product and its FW v5.3.3 also does not allow a 50Hz to 60Hz switch since China is a 50Hz country. I'm guessing that the DS-2CD2335-I is a Chinese market only IPC too, hence no 50Hz to 60Hz switch.
Have you haven't noticed any issues with using the cam at 50Hz, I haven't?
As a side note, I'm not impressed with the 3335 4MP camera. Day quality is great, better than the 2332, but night quality is very poor. The motion blur makes face recognition impossible. If you increase the shutter speed much beyond 1/150 sec the built in IR light just can't produce enough light.
It is good to know that the 3335 camera retains the excellent metal mount. Maybe the 50/60Hz thing is a firmware only issue? You could have tried whoslooking's v5.2.5 downgrader.
I did try smart ir on and off on the 3335 but the motion blur was horrible standing only 15 feet from the camera. I played with shutter speeds, WDR, etc., nothing short of more light would improve the situation. The 2332 had no issues so I went back to the 2332 (for my front door).
I wouldn't do that if you want it to work still.