Holley Jet numbers are all flow based. You can find two jet sizes with the same diameter bore because the lengths of the bore and the entrance and exit shapes are different.
A two digit standard jet is allowed 3% variation in flow.
Three digits ending in 1 or 3 were used by factory calibration, and I too have found H on a jets installed by the factory.
Three digits were also used for close-limit jets with only 1.5% flow difference allowed.
Last are three digit metric jets used on model 4360 and 5200 carbs.
(ref. Mike Urich & Bill Fisher Holley Carburetors and Manifolds HP Books. Holley part No 36-73, 1987 revised)
