Furthermore, FTDNA has got its primers for L277 working so that we can confirm that both L277 and L584 are subclades of R-Z2105.
Kits that turn out to be Z2105+ should consider ordering L277 and L584.
However, both L277 and L584 have varying frequencies according to geography. For example, few men with British Isles ancestry are turning up positive for either SNP whereas many (in some cases most) men with SW Asian (or Jewish) ancestry are turning up positive.
However, kit 176123 in L23EE Type has already tested negative for L584.