I believe it is zinc phosphate, but not certain.

From the pictures I've seen of preban SG550 and the recent JDI imported SG55x series, the bolt & carrier looks nitrided.
According to the online sources I found, the barrel should be nitrided but from pictures it looks more like a dark parkerizing on the outside.
The Wikipedia page does say other components should be phosphated, but I can't find any info on this.

Barrels vary depending upon production year. All Swiss barrels are nitrided. Some early ones look to be black oxided, while military stg/PE90 barrels and later production guns have a darker phosphate finish applied. Of the multiple 551-1 LB barrels I’ve owned, they all appear to be manganese phosphated, as do the barrels on the 553 imports (M&M and Exeter....no personal experience with JDI). 551 short barrels are a bit lighter, as are PE90 barrels. 552 barrels, I’ve seen both darker and lighter phosphate.

What I am mentioning as zinc oxide is the receiver. The receiver interior is not painted with Ilaflon and appear to be zinc phosphated.

Thank you for the info.
I didn't know you can apply phosphate over a nitride finish, interesting.

It's possible, that was essentially the old Glock finish that everyone loved so much.

Takes an acid treatment to remove the oxide layer from the "quench" step if was salt bath nitrided. If it was gas nitrided might be easier.

If you knew you were going to do this, could probably pull the part from the last polish step of nitride then go to parkerize.

Thanks josh. Wealth of knowledge as usual.  My exiter imported 553 seems dark black and very resistant to wear like I know nitride to be.  My jdi imported 553lb seems more grey and gets bright spots when scratched or rubbed.  More like I know parkerizing to act.

How can one tell what the finishing process is on various metals?  Is it a texture characteristic?