I bought an M4 upper receiver assembly for my RR from a friend of mine. He told me it was all Bushmaster. The 14.5" barrel is marked "B MP 5.56MM 1/7", but the flat-top upper receiver, the bolt carrier and bolt have no markings on them at all. Is that typical for a Bushmaster upper?

I believe that "BMP 5.56mm" is typical of Bushmaster markings, although they mostly provide 1/9 rather than 1/7 barrels.  I went through the same thing a few years ago and I do know that their uppers do not say "Bushmaster" anywhere.  There are several things to look for though such as chrome lined bore, double-pinned bolt carrier key, etc.  Go to the Bushmaster website - it has a whole laundry list of identifying features to help you with this question.

Check for what looks like some grey paint on the inside of the upper.  That is the dry lube, and I believe it is only found on Bushmasters.

I don't know abouT Bushies, but I don't see any MP markings on any of the other aftermarket bolts I've examined in the past couple of years.  That's why I "retired" a practically new Rock River bolt, and replaced it with a GI surplus C MP assy recently.  The Rock River, and other aftermarket units I've seen are NOT radiused at the edge of the hole. You can darn near cut your finger on some of them; the edges are so sharp. [:D]

Though I own an SP1 Colt, I'm not Colt fanatic, by any means----except when it comes to the innards. Bolts, Carriers, Triggers and Hammers in my 3 ARs are all either Colt or military contract parts.  I'm not saying that nobody on the aftermarket makes decent internals;  only that when I have the Colt stuff in them, I don't have to worry--------------as much[:D]