It is my understanding that the so called Double valve stems (fill valve plus relief valve) were installed as part of a BFG safety campaign in 1973+ timeframe. This followed several injuries and fatalities when incorrectly filling the collapsible spare using a garage air compressor as opposed to the canister, and was pushed through by the Federal authorities at the time. BFG filed a patent application in that year and was installing them 1974 onwards, including retrofitting them to existing space savers.
This safety recall was carried out across all makes equipped with BFG collapsible spares (AMX, Mopar etc) not just Ford. Here is a copy of it sent to an AMX owner.
To get back to the original question, you need to look at the wheel rim stamped date code and also any date codes on the tire. The DOT mandated identification number has the last 4 digits as a date code, and started being used in 1971. No DOT number means it is pre-71 and I have yet to find anyone who can decode those numbers.