I have a few Ar-15's, and I have bought what I thought were all Bushmasters, but I noticed that each has a different Marking on the right side of the Handle just forward of the A2 rear sight.  One is a arrow symbol, one is a BM, one says FN and one is blank.  Does anyone know where I can find an online guide to these markings?  Does this mean that just the lowers are Bushmaster and the Uppers have been swapped out?  Thanks for any info

Different markings on the upper recvrs don't mean very much by themselves. I have three factory Bushmasters and all three have something different. It's the barrel markings and lower rcvr markings that mean the most. See a very few places make the raw forgings and sell them to the various manufacturers like Bushmaster and the like. Colts that I've seen are C marked only. That may be the one exception.

Thanks,
Mark

Mk1iii has it right.

I have seen bushmaster factory rifles with all kinds of upper receiver markings.