143 is prefix of MR223, so you have bolt that originally was made for MR223.For the record, my bolt starts with a 143 prefix. In any case, I'm sure all US MR556 receivers start with a 241 prefix serial numbers as I believe that's a model prefix.
The box serial number matches the lower receiver so that's a non-issue. The internals seemed pretty minty fresh though. No copper on the feed ramps or brass on the bolt head (although to be fair, it's not that hard to remove). However, I'm still not inclined to believe it was anything more than finger banged on a rack somewhere, it just seemed there was an excessive amount of carbon build up on the piston. When I got my SL8, there was almost no evidence of it being test fired, though I'm sure it was.
This still wasn't the main point of my question though, I was more concerned that serieled parts which may have matched my rifle were somewhere along the line swapped out or replaced. Maybe they had two MR556's stripped out next to each other comparing internals and put the opposite bolt back in with my gun. The whole bolt explanation makes sense though so I think at this point, problem solved.
Now one thing I swear I remember hearing is that the MR556 uppers are serieled to the lowers, is that true, and where is the number? I've examined the upper high and low and can't find any serial markings. Again, I just want to make sure that somehow original parts didn't get swapped on me.
ETA: I was going off of memory when I said 168 for the bolt prefix. I double checked it this morning - it's 143.
ETA II: When I unboxed it there was also a small gouge on the warning label (~1 cm long, .25 cm wide but deep enough to form a groove and expose bare metal) on the front of the magwell and also the oprod spring was rusty. What should I do? I don't need a cherry gun, but it makes you cringe when you spend 2k+ on a rifle and you find that right out of the box.
When I browsed HK USA printed catalogue all MR556 and MR762 were in fact MR223 and MR308 - and those have serialized lowers and uppers. So photo on manual probably is MR308.I've never heard that the upper has a serial number matched to the lower. Neither my MR556 or MR762 have a s/n on the upper. The beauty model on the cover of the MR762 manual does have s/n on both the upper and lower, but both have a US prefix, so it may be some kind of prototype used just for the manual pictures.