Bushmaster's upper receivers usually have markings on them, but they aren't BUSHMASTER's markings. The markings are die markings from the dies used by the aluminum forging company that made the raw forging. These companies supply raw forgings to most of the AR industry, so it isn't unusual for the same forge markings to appear on rifles from many different manufacturers. For example, we've seen Anchor Harvey forgings used by Colt, Bushmaster, Olympic Arms, and others.
What counts is the machining and finishing of the upper, and there's no way to know who did that unless you buy directly from the company.