Carter Thermoquads ~ 'More General Information'

Pretty sure there was a 950 Holley 3 bbl that had conventional boosters in the secondaries.

yes, the 950 3-barrel had booster venturis. I just picked one up for $65
the 1050 3-barrel had secondary discharge tubes, without booster venturis. that's how they picked up an additional 100cfm, making it a 1050 cfm.
the trap door carbs were a 1960's Holley racing carb and short lived fad. they were surpassed by the late-1960's DOMINATOR Holley carbs first used on the NASCAR BOSS 429 engine. truth be told, the early 850cfm vacuum secondary L88 Chevy carbs were better than the trap door 950/1050. Which is why Chevy put them on the 427 Vette motor, instead of a 950/1050.
a trap door Holley was nothing more than a prototype 4150/4160 850 vacuum secondary carb, with bigger secondaries- the primaries are the same size as a modern Holley 850- the single big oval secondary throttle plate gets a little more cfm by eliminating/opening up the wall area between the rear throttles, for 100cfm more airflow.
the problem with a trap door is, timing the secondaries. they either open too fast and bog, or open too slow to the point a standard 750/850 double pumper or vaccum secondary carb is better. the big trap door vacuum diaphragm is a nonstocked discontinued part, and to my knowledge tuning parts for it are not available new. you have to use the cut and try farmer method to tune them. the trap door also uses a special secondary throttle actuating lever, that bolts onto the secondary throttle shaft, and connects to the vacuum diaphragm rod. lose that little part and you're screwed, you have to make a new one from scratch using a standard Holley vacuum secondary lever, and weld a horizontal extension on it.
the one I have was converted to mechanical secondary linkage. that takes a bad situation and makes it worse. no doubt it will bog badly, as the secondary side has no accelerator pump. now the secondaries will open very quickly and it's a sustained high-rpm only carb, which is exactly what it was designed to be in the first place. The trap door was a stopgap design, until Holley developed the modern DP and vac. sec. carbs with 4 equal sized bores. The modern Holley 750/850DP was cobbled up and designed in Smokey Yunick's shop at Daytona Beach, Fl. during the 1960's for NASCAR. He wrote about it in his autobiography. He had the original epoxy/aluminum/tin plate prototype, lent it to Bunkie Knudsen at Ford, and never got it back. John DeLorean also looked at it, and said it was a good idea while he was General Manager at Pontiac, and later at Chevrolet. This was during the pre-Quadrajet era when the biggest 4GC Rochester was only 700cfm. They were at 421/428/455 Pontiac engines, and headed for 472/500 Cadillac engines, and needed more carb capacity. That's what led to the 800cfm Qjet, 800/850/1000 cfm Thermoquads during late-1960's, early 1970's.