There's a selector switch that goes between "A" and "B" with B being the lower numbers. Which is used depends on the loading of the airplane. In most cases I've seen in the past few years, the switch is safety wired to "B". In either setting, the VMO is increased for each 10,000 feet altitude until it matches the MMO. I think all 727s are like this, at least all that I've flown are.