1)The new model(151 and up)serial number prefixes the Security-Six was only cataloged with the adjustable sights. Both the Service-Six and the Speed-Six had the fixed sights. The Service-Six had the same aquare butt as the Security-Six and the Speed-Six had the round butt.
2)The old model(150)serial number prefix Security-sixes initially were offered with both the fixed and the adjustable sights. However the early "fixed sight" Security-Sixes went away as their name was changed to Service-Six-thus leaving only the adjustable sight Security-Six in place. Then later on, the Service-Six was eventually stamped as a Police Service-Six.