The multi-link slide positioners (as found in the 627/022/069 etc. series) and the screwed-down covers were the 'signature' of the PD-style carbs. While not as tightly jetted (nor as well manufactured) as the 'real' PD carbs that came after 1975 in all the SOHC4 bikes, they were the forerunners of the PD family, bred from Honda's roadracers (and are seen on the prototype 750 bikes as well). They are much more expensive to manufacture, which was likely where the 'roundtop' 750 and 'copycat' (i.e. those press-in O-ringed jets) versions in the early 500/550 series got used instead. The jetting family of the early 500/550 carbs came right from the successful CB350 series jetting, so Honda/Keihin didn't have to make a fully-engineered jetting system for the early mid-Four. The showroom prices were Honda's driving force in those days: the 500 debuted at $1095 (the 450 twin was then $995) and became $1195 by the 550K1, but jumped to $1345 (or $1395 west of the Mississippi River in the USA) when the PD carbs with the more accurate screw-in mainjets appeared. Honda tried to offset some of these costs in other ways (like by cheapening the swingarm pivot parts, using wheel bearings with just one side sealed, reducing wiring and pushbutton controls, and 4-1 exhaust), but it still meant more cost due largely to the carbs. This happened with the 750 at the F2/K7 series as well. Cutting threads in carb bodies is not a cheap thing to do!
In terms of mixing performance, the later PD carbs (with the screw-in mainjets) are much more accurate mixers, although they came from the factory set quite lean for emissions reasons, so they are often perceived as not making as much power. These were also the first carbs on the 750s that would not over-feed during throttle shutoff, which caused lots of fouled sparkplugs in city riding, especially. Right from the beginning of the 500/550 carbs we had troubles with the O-rings leaking like the CB/CL350s had done after about 2 seasons of riding, and it was because of the leaking O-rings on the mainjets - or sometimes the mount where they plug in would warp, making it leak fuel. We replaced lots of those little O-rings, sometimes with softer durometer rubber (if the hole was warped) to make them seal again. When they became screw-in jets in the 550 this problem ceased altogether.
