I think there's some confusion here. The grid extends east to west, and only includes primarily north-south streets on the same angle as the Back Bay grid. It skips from Hereford to Ipswich on the other side of Mass. Ave. Ipswich is sometimes parallel to Newbury, but only because it had to run along the railroad tracks to get to the Charlesgate. The "relevant" parts of Ipswich, on the right angle, only run between Charlesgate East and Boylston and Van Ness and Boylston (so it technically comprises two streets in the system). After that, Jersey and Kilmanrock continue the system west, and I can't find any L street that would work past there (Longwood Ave. would be stretching it for several reasons - it's both an Avenue and the angle is slightly off).
Newbury, Marlborough, and Lansdowne are all east-west streets, in the wrong order, and are on the wrong side of the grid for them to work.
EDIT: considering Charlesgate West was originally Jersey, there's definitely an argument to be made that Kenmore is the real continuation, followed by Lansdowne (if it connected or was intended to connect to Beacon at the right angle) and maybe even tiny Maitland. All three are British aristocratic titles, the one requirement for these streets. The doubling-over of Ipswich may have "reset" the system on the other side of the railroad / Pike so that there are actually two systems. HOWEVER, Jersey/Kilmanrock follows the Back Bay stipulation that the streets alternate two and three-syllable names, whereas Jersey/Kenmore does not.