They were original equipment on several "specialty" cars, such as the Baldwin Motion Camaro, Some of the Yenkos cars and they were also used by NASCAR. There were two sizes from Holley. The 950 and the 1050. Both were vacuum secondary carbs, although there was a kit consisting of two half moon gears that attached to the throttle shafts that made them not only mechanical, but 1:1 in regards to the opening ratio of the primary and secondary barrels. There was also an extension with the kit for the primary pump squirter that transferred fuel to the secondary barrel as well.
The idea was to do away with the "dead space" between the secondaries to allow for more flow while still retaining the 4150/4160 base plate. This allowed the cars to run in classes they normally could not if they had a dominator. The carburetors worked incredibly well. I am gonna find out when I get my 383 together because I have a 950 rebuilt and ready to go.