CB550K '77-'78 KeiHin PD46 Mechanical Slide 2nd Gen
These carbs are basically the same style for both years of the CB550K Four '77-'78. They can be interchanged as-is with no mods to the bike or carbs. However, they will not interchange with the first gen 500-550 carbs without modifications to the bike. The easiest way to tell the two years apart at a glance is the main throttle return spring. The '77 has a conventional spring that sits 90 degrees to the throttle axle and the '78 has a torsion spring that wraps around the throttle axle. However, there are many other differences in the two years that you need to pay attention to when rebuilding. Below are pictures of both years.
Pictured below are both styles of the CB550K '77-'78 so you can see the differences. They will interchange with each other, but are not the exact same carb.
'77
CB550K------------------------------------------------------PD46A
'78
CB550K------------------------------------------------------PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C
'77 PD46A
'78 PD46C