Though the AKRF study published this map, it failed to provide the "key," which indicates that the dashed line represented "buildings communicating." The significance is explained in Appendix II of Volume X, No.1 of the journal Village Views, published by the Society for the Architecture of the City:
The plate showing Duffield Street connects the rowhouses in the terrace running from 129 to 143 Duffield with dashes across the front areaway vaults. 141 Duffield St. was the original numbering of the Truesdell House, now known as 227. This is exactly as described by residents in oral histories: they said years ago the basements were connected through vaults at the front under the sidewalk, connections now vestigial, due to demolitions and alterations.
We've annotated details from the map and key: the yellow area highlights the dashed lines and 141 Duffield St. (now 227 Duffield) is circled in red.
Above: detail from Plate 28 of Brooklyn Perris Map.
Right: detail from key to Brooklyn Perris Index Map Volume 1.