Ron, what I mean in refering to a progressive sled is one where the weight starts moving as soon as the sled starts moving and progresses forward (onto the pan and off of the wheels)  constantly relative to forward motion of the sled.  That is the entire operation of it.  The pan starts out on the ground with the front of the weight transfer sitting on it. This is in contrast to the racing sleds which move the weight progressively up the rails but then add some sort of artificial fiddling, usually in the form of a lock which holds the back of the pan (usually containing some sort of teeth or bars to dig into the track) off the ground until some point when the car passes a trip and then drops it.  The result is that the load is no longer applied progressively from start to finish, but rather the sled runs light up to the point where the pan drops at which time no one in the class will have enough tractive ability to continue to pull it.  Whoever is going the fastest when the pan drops wins.   You can run a 10k farm stock against a 6.2k 2wd pickup and set a race sled such that the pickup will win.  With a fair sled the pickup would never move if the sled was set to stop the 10k without going out the gate.  I've never really figured out what the appeal of the racing sleds is other then that they allow mediocre sled operators to keep nearly any class bunched up so that everyone stops in the last 75' of track. 

Whoever successfully patented the mechanical transfer sled was far from the first to build it.  I am a member of an organization here which had a sled in operation for some 15 years before the patent business ever came about, and it while it was an early one, there were numerous others around running at the same time.