Well . . .
cannot is a very strong word! Your statement would be a good rule . . . IF S&W followed rules.
If we're talking about general production (i.e. catalogued) items) . . . and we accept the premise that a Masterpiece with less than a 6" barrel with Baughman ramp front sight IS a Combat Masterpiece . . . then your statement is still a good rule of thumb.
But then there are the special orders and what I call "market test" special production runs.
Under special orders for example, in 1957 S&W produced a 4" Heavy Combat Masterpiece with Baughman ramp front sight for the Kansas City Missouri P.D. And there were orders for other agencies including the Border Patrol that have been documented.
Under special production runs, we know S&W produced a group of 4" Model 14-3s with Baughman ramp front sights in 1971 that were shipped to several well-known S&W distributors for what I can only believe was a market test . . . i.e. they were not for a particular law enforcement agency or department. A picture of one such example is below.
And then there were the 4" (Extra) Heavy Model 14-2 Dayton Guns . . . but then that's a whole different story.
Probably much more than you wanted to know . . . but whenever we use words such a always, never or cannot . . . you know there will be exceptions when it comes to S&W. But that's some of the fun of studying and collecting Smith & Wessons!
Russ