This is the 'variable ratio' power steering box, Saginaw manufactured. The Variable Ratio steering box was part of the Competition Suspension Package available optionally on all Cougars or part of the GT or 351CJ Performance Group.
This Variable Ratio steering box should have come standard with all 1971 - 1973 M or Q code 351-4V or C or J code 429-CJ equipped Cougars.
Sometimes this tag number could also be SPA-T, SPA-V, SPA-AD, SPA-AF,