This mark is part of a First-Order Triangulation network set up for the City of Boston by the WPA from 1935-1941. Information on the location was found in Report on a Geodetic Survey for Boston, 1935-1941 (WPA Project Numbers 65-14-5744, 665-14-3-952, 165-1-14-259), published Nov. 1942.
The network consisted of 65 stations spread throughout Boston, spilling into neighboring municipalities. Many of the stations had local transfer triangles, consisting of one or more disks - this disk is not one of the 65 stations, but a transfer point.
The disk is a transfer point for the station CHAHY. Station CHAHY, if it still exists, sits atop the Schoolhouse on Monument Square condominiums (30 Monument Square; at the time, Charlestown High School).
The aforementioned publication has this to say about the transfer point itself on page 51:
"Ground Transfer Point, 'Chahy', CPB Disc No. 39 - (First order traverse No. B3/0)
Description: This point consists of a C. P. B. disc set in curb on south side of Monument Square near Pleasant Street directly in front of house No. 38 on Monument Square.
The text gives specific measurements, but the area has changed some since 1938, so I will give new measurements. The disk sits near 37-39 Monument Square and is embedded in the curb on the south side of the street. It is 53" from a 16" tree, 100" from a gas cover, and 63" from a second gas cover.
Table II (p. 176) of the book gives the Massachusetts State Lambert Projection Coordinates (based on the C&GS adjustment, 1941) of this station as (718268.31, 501633.28).