The Schwinn Paramount P15-9 was released in 1970 with a Campagnolo gran turismo derailleur and a 31T rear max freewheel (I think it shipped with both Regina and Suntour freewheels).
http://www.geocities.com/
The problem imho in the late 1960's wasn't that of making larger freewheels, rather, it was a problem of getting decent derailleurs to shift a massive freewheel. "The Dancing Chain" book by Frank Berto would be an authority on derailleurs in this time period.
All of that changed with the introduction of the steel Suntour-7 slant-pantograph derailleur in the late 60's (and VGT alloy model a year or two later), and 30T+ freewheels really took off, especially on japan-made mid-level bikes in the latter half of the 1970's.
http://www.bulgier.net/
So, Schwinn : 1970 30T+ Raleigh : 1979 30T+
- Don Gillies San Diego, CA
P.S. I'm betting 28T max-cog freewheels are from the 1950's or before.