I picked up an upper that is complete­ly unmarked except for the (BI) or possibly (BFI) on the FSB.

Guys over on the CQBR forum said it looks to be a 11.5 inch barrel, A2 upper with an A1 forward assist. I cannot locate the barrel twist rate or cal­iber markings anywhere either.

Im looking to replace the standard ­carbine handguards with either a FF or railed­ option, but one that would keep with the ret­ro aesthetic and work with the FSB and A2 upper.

https://imgur.com/a/ZAklWwZ

It appears to be a DelTon A2 Upper (older DT A2 uppers sometimes didn't have forge marks) with a 11.5" Heavy Barrel with standard A2 FSB.  Most likely 1/9 twist and does not appear to be chrome lined.  DT Heavy Barrels have that small rise close to the barrel extension and then slopes down a little all the way to the FSB shoulder.  I thought DT marks their barrels, but maybe not on the 11.5" barrels as usually they are marked at the end of the barrels.  I think ER Shaw makes a similar barrel or could make them for DT, but I thought DT cuts their own barrels.  Who knows anymore and could be made OEM for several different companies and that's why it's not marked.  Possibly could of been a 16" heavy barrel and chopped to 11.5" cutting off the markings on the end of the barrel.  Anyway, DT BCGs (most likely an auto carrier) were not marked and nothing usually marked on the bolt either, but are supposed to be HPT/MPI tested.  
DT barrels are nice shooters.  I have one complete DT upper and several other DT barrels and no complaints.  

That's a Cool Upper!  But, of course, someone could have hodge-podged a bunch of different AR parts together for a Franken-build.  Be sure to check over everything.

You can put a KAC RIS/RAS, KAC VFG, some KAC Rails and possibly an older SF951 light on it with a standard M16 sling.  That would keep it more retroish-looking.  Then you'd have a cool A2 Commando ... kinda' like the Mexican military/police used to use.  Or you could just put a set of M4 hanguards on it instead of the skinny handguards and call it good.  Check out the EE for stuff, be patient, and you'll come across good deals.

Also check out the A2 forum section.  They'll give you a bunch of suggestions.  We like spending other people's money    
Good Luck!