I don't know much about the US made 556 series. Are their stocks interchangeable with the Swiss 55x guns. Found a stock advertised as 551/556 but not sure if this Swiss made.

Any of the 556 guns that use the traditional 55x folding stock could use a Swiss stock. I think that’s the 556 “Classic”, 556 “Classic” SWAT, 556R Gens 1 and 2, and the 551A1.

The original “fish gill” 556 with the M4 stock, the DMR, and the 556 pistols all came with lowers that were setup with the buffer tube lower that cannot use a Swiss-style stock. Sig used to sell a US-made conversion lower but you may need a time machine to track one down now. JDI may import lowers but be prepared to spend a small fortune getting one.

I swapped stocks between my Sig 556 Classic SBR and my Swiss made Sig 551LB.  The Swiss made stock went on to the Sig 556 Classic SBR without issue.  However, the US made Sig 556 Classic stock had to be fitted slightly to go on the Sig 551LB.  Now that they're installed, only someone really in the know can tell a difference.

Completely interchangeable if you’re talking about the Swiss pattern stocks. The US (or India?) made stocks are not nearly as quality as the Swiss and are easy to spot if you know what to look for.