If you've cleaned your gun really wellsince you stopped using Bore Butter, then I doubt if that's what it was.
If it was Bore Butter, then the next time that you clean it with hot water, you probably won't see it again.
If it's from the 777, then you'll see it again next time too and you'll know more about where it came from, unless it's fromburned plastic or something else that you've put into your gun barrel.