bought a mystery 850 double pumper...

I guarantee it's 1.56 venturies 1.75 throttle plates.
the story is 850cfm w/downleg boosters 830 cfm w/annual boosters because the annual's are bigger and restrict the air flow a little. Now they just rate them 850 because the actual cfm varies from CU.IN and RPM. If you read the specs it says 800-899 cfm rating.