Sounds like the 'splintered A' mark is an Alcoa Forge marking.
You see, every upper and lower receiver forging comes from any of several aluminum products companies. The commercial forging divisions of the companies are the ones that actually produce the raw forgings that are used by every AR manufacturer. Some of these will be specially marked for the customer, and others carry only the aluminum company's marks. Either way, the marks are in the forging dies so every part forged in them will carry this mark.
The only thing you can be sure of from these marks are which company made them (if the marks aren't milled off in the process of finishing the part) and sometimes, which firearm company they were intended for. But undoubtedly, the firearms companies do some shopping around. Colt usually deals with Alcoa for forgings, but if Diemaco-Kaiser (a main Bushmaster supplier) has the better prices this month, Colt may buy D-K's, and next month Bushmaster may buy Alcoa's products. So, I wouldn't count on those markings being 100 percent accurate 100 percent of the time.
I've heard that the name for the hard grey coating Bushmaster applies to the inside of their upper recievers is called something like "Michelson hardcoat" or something similar. Has anyone got any info on this stuff?
I know that it's TOUGH stuff. Most of it stayed on my upper receiver when I stripped it, bead blasted it, and re-anodized it. Very little of it came off.