It could be that someone removed the secondary metering plate and replaced it with a metering block, like in the front. OEM jetting is 72 primaries and would be 75 or 76 for the metering block in the secondary position.
Originally had a power valve up front but not in the secondaries.

Good all-around carbs. Tuned right, they might be only 1-2 tenths slower than a properly tuned 750 double pump carb.