The R6551 is going to have a threaded barrel with an A2 flash hider but no bayonet lug. Except for a few rare instances, all of the “blue label” Colts had the bayonet lug filed down for “PC” reasons.

I think a NIB R6551 would probably go for about $1000-1300 depending on whether the end user was planning to keep it as a NIB collector’s item or take it out and use it (like I would ).