I dont see a trunnion in your picture.
It doesn't surprise me that RCM trunnions aren't affected by nitriding, considering the way they are heat treated and the 52 HRC hardness they come with. But if you don't know that you have an RCM trunnion, it would be a bad idea to assume that the salt tempering isn't going to hurt a part of the gun which isn't replaceable in the normal sense.
So first you were kicking and screaming that its made out of O1 tool steel and it will be ruined by nitriding. Then you say that with 9310 you dont want to guess at what would happen with nitride, then you say you aren't suprised that they are not affected by nitride. Which way is up dude?
Factory HK or contract (POF, EBO) trunnions are likely something like O1 because they have a hardness of 60 at the back and lower around the barrel. It would be bad to temper that.
RCM trunnions are 52 HRc throughout and made of a 9310, a steel that is hardened almost entirely by carburization since the base metal has only .1% carbon. If the company that makes 9310 parts says further carburization at +900° isn't going to affect it, then it shouldn't.
Who makes PTR trunnions and how? Thompson Center? We know they use a mix of surplus and new. http://www.ptr91.com/news/ptr-trunnion-issue-details/
Then there's Michael's Machines and anyone else that is quietly in the game. What steel and what heat treat? How do you tell an RCM trunnion from a PTR from an HK?
But we don't know what the OP's or Brethern's trunnions are made out of, so to suggest that nitriding is a universally safe choice for any "clone" is off base, because if it isn't an RCM or other 9310 trunnion you are ruining the heat treatment. And the fact the HK ones are hardened they way they are suggest that they aren't 9310, and HK is not sharing its heat treat or alloy information.
Brethern's trunnions are RCM.
I just tested my hk mp5f trunnion and it is at 57rc near the roller area.
That's good that the company getting HK clones nitrided has RCM trunnions. But I would seriously avoid using nitride if you don't know what it is going to do to the important steel parts that are heat treated.
Get to the bottom of it for the sake of the very interested parties. But know the terrain. On another forum, a similar question of quality control arose. And it hurt everyone but the consumer. Lawsuits against people posting mainly but not 100% statements. Manufacturers who could not verify their claims. Ugly.
Since I have a handful of HK clones with a mix of RCM and German surplus, nitriding is out.