I think the idea was that you could cut the rolling resistance by raising the center of the tread. The idea had been around for a while with Dunlop producing a tire (Road Speed?) which had a raised center section (but with a groove at the center) during the 60s it had a Japanese imitator branded ''Million'' .These were the first mixed tread pattern tires (there was a raised diamond pattern to the sides of the center ridge)I remember seeing.I recall the Dunlop casings as being prone to rim cutting. Under the ''old is new'' I see a current tire with this tread pattern being called a rain tire. During the mid 70s raised center tires were produced by Goodyear and the Japanese manufactuerers and sold not only by Specialized but Schwinn, Avocet and others.These I think were marketed as touring tires. It turned out to be a poor design as the edge of that center section would catch road irregularities and produce an unstable feel when transitioning from pavement types.They were particularly tricky if you slipped off the road edge and then tried to ride back onto the pavement.
I suspect Garths ''Squishy'' sound is related to the rubbers thickness at the center of the tire.The treadless tires came after this period and signaled the ''We sell the lightest tire'' phase of the bike business also called the ''lets lie about the actual size of our tires''
BOB FREITAS coffee kicking in here in MILL VALLEY, CA USA
Dave Staub could have added more here and you could probably search for his input in the archives (Dave, we miss you)