This is the master square head key blank.  The master key blank is known to open all locks.  This is a "Yale" key blank brand. From 1970 VIN 9001 up to 1980  

This is the square head key blank U.S.A. reproduction (Master Key) (1 7/8"long).  1955-1965 Silver Cloud & S-series, Phantom V, Continental 1966-1969 Shadow, T-series, Corniche...

This is the Master Square Head Key Blank (1 7/8" long)  1955-1965 Silver Cloud & S-series, Phantom V, Continental  1966-1969 Shadow, T, & Corniche

This is the round head key blank (1 7/8" long).  Round key head (limited access for valet) fits doors and ignition only.   1956-1965 Silver Cloud, S-series, Phantom V 1966-1969...

This is the key blank for all Pre 1955 year models.  This is an aftermarket brand.   Pre 1955 Models, R-type, Silver Dawn, Silver Wraith  

This is a key blank (for you to have cut to your car's key) limited access. 1966-1980  Silver Shadow, T-series, Shadow II