Though am not sure about that. Looking at the rest of the map, taking this feature from where it leaves Pinfold stseems to start on the edge of the map (though may merely go further off the map), up the present Colmore Row, then turns down Stafford St, turns off Stafford st to the east and runs behind the houses in Stafford St to Coleshill St, down Coleshill St for a short way, then off Southeast to cut across Masshouse Lane, then down moor st, turning off half way down.It then seems to change colour to red dots, going south taking a not well determined path through the houses between Moor st and High St, crossing High St south and close to the junction with New St, going behind the southern buildings in New St to Peck Lane to Pinfold st, and then behind the buildings of Pinfold st to where we started. The colour of the dots changes back to black at the bottom of Pack Lane.
So it looks a bit like some sort of boundary.