1 oz 1st class stamp in 1971=8 cents.
1 oz 1st class stamp in 2007=41 cents.
512.5% increase

CPI in January 1971=39.8
CPI in January 2007=202.416

508.9% increase.

Looks like Slate is wrong on this one. I can't remember which year of the 70s I looked at last time where I got the 50% number.