21821 is a postalcode and its consensus geometry is derived from uszcta.
{"mz:is_ceased":0,"geom:latitude":38.166712,"wof:name":"21821","counts:names_total":0,"wof:placetype":"postalcode","mz:is_current":1,"wof:hierarchy":[{"continent_id":102191575,"country_id":85633793,"county_id":102082187,"locality_id":85949569,"postalcode_id":554780651,"region_id":85688501}],"mz:is_deprecated":0,"lbl:latitude":38.164594,"geom:area":0.004203,"geom:longitude":-75.92231,"wof:id":554780651,"counts:concordances_total":1,"geom:type":"Polygon","wof:lastmodified":1611185168,"wof:belongsto":[102191575,85633793,102082187,85949569,85688501],"wof:concordances":{"gp:id":"12766742"},"wof:repo":"whosonfirst-data-postalcode-us","src:geom":"uszcta","wof:country":"US","counts:names_preferred":0,"gp:parent_id":"12588694","geom:area_square_m":4.0859060652315E7,"wof:tags":[],"counts:names_variant":0,"wof:breaches":[],"edtf:inception":"uuuu","iso:country":"US","counts:names_languages":0,"geom:bbox":"-75.963377,38.123627,-75.860559,38.207214","wof:superseded_by":[],"wof:supersedes":[],"lbl:longitude":-75.916659,"wof:geomhash":"ffab07c5cb3730515df297cb1768eb75","wof:parent_id":85949569,"wof:placetype_names":["postalcode"],"mz:hierarchy_label":1,"wof:placetype_id":470996387,"edtf:cessation":"uuuu","mz:postal_locality":"Deal Island"}