Primary metering rods can be put into two groups, the long ones for the early carbs and short ones for the later carbs with the APT system located in front of the PP. (1975 and early 76 aneroid located APT carbs used short metering rods as well, but no provision to raise/lower them).
There are several secondary stamps for each group, and they tell us the diameter of the largest part of the upper section, and type/amount of taper/tip size.
Most early carbs used a either single step or "B" series which has a slight taper to the upper section, then quickly tapers to .026". Some Oldsmobile carbs used a tapered two step early metering rod, but they are pretty rare.
Later short metering rods were J, K, L, M and P series, with the exception of the later motorhome carbs which were full taper with .036" tips.
J and L were tapered, K, M and P are "stepped". K rods have .005" of taper on the upper section then step to .026". M series have .010" taper then step to .036", P series have .010" taper then step to .026".
I like and use J and L series for the later carbs with 4 MAB's, as they are basically full taper, slowly on the upper section, then quickly to .026". I don't like or use "K" metering rods in most cases, not enough adjustment. For the SMAB carbs (except the 1977/78 Chevy truck units) I use "M" series in most cases........Cliff
