That is a big gap, Mastershake340. But to your point that probably is the original space-saver spare in your T/A. I'm sure there are probably many examples of this if we could take a cross-section of cars built in the 1970 model year and develop a chart of space-saver rim date codes vs. vehicle SPD.

I believe the 30-60 day is "general guidance" and not an absolute rule with space-saver date codes relative to SPDs.