CD Stromberg O-Ring for Bushing Retaining Screw and Float Bowl Plug in 125CD 150CD 175CD      P/N 019658

0.61” I.D. [15.5mm]
0.81” O.D. [20.6mm] 
0.010” thick [2.54mm]

This O-ring [20.6mm O.D.] is the large diameter one used to seal the bushing retaining screw on ealier series, 
OR the Float chamber sealing plug located at the base of the float bowl on 150CD and 175CD

Number 28 in illustration below

The other O-rings used in these carburettors are 016268 [11.2mm O.D.] number 27 in illustration below [located on the Orifice adjusting screw]
P/N 019657 number 60 in illustration below [located under jet orrifice bushing]