S or $
This cover on Roosevelt Island is very much like covers seen in Yonkers and the northern Bronx that date from the 1870's, when both areas were part of Westchester County. The Roosevelt Island cover is near to the location of the old Woman's Alm's House and Maternity Wards which opened in the late 1870's and which probably had some sewer service, This cover then could then be a relic from that time or earlier.
The distinctive wagon wheel design with 16 spokes is often associated with drains or sewers. This particular design with rings of 7, 5 and 4 vent holes is seen in several spots in the Bronx and in Westchester County
It could be $'s.
This outlier is on West Road just north of the subway station on Roosevelt Island.