Here's an original Carburetor hold down bolt (1971 340, Thermo-Quad). Never off the engine. Ask AMD for one of these and you'll get a bolt with an "H" head marking.

This engine has fastener details I've seen on a number of other 71 340's it's not an anomaly. I've NEVER seen one (known original) example of a 71 340 with the "H" bolts on the carbs OR the exhaust manifolds yet many sellers of reproduction hardware sell the "H" heads as "correct" for this application, likely because it's all they have.