The plug or spigot ferrule system is a traditional ferrule system used by many rod blank manufacturers for a long time. I think the J. Kennedy Fisher Company introduced it back in the 70's and many other companies have continued to use it to the present, including Diamondback, Winston, Scott, and two of the custom blank builders I have do blanks for me. This is a desirable system for the small blank builder since single mandrels can be used to wrap the whole blank. The resulting single section can be cut in various configurations, plugs glued in place, and rods of many sections produced. The six piece rods along with several other models in my Presentation and Signature series of rods this year use this system. It is important to understand that a 1/8" to 3/8" gap will remain when the sections are put together. This "gap" allows for wear. You should never try to push the sections the whole way together.
My short six piece rods use this system for all but the top
section, which is a tip over butt ferrule. That is the key to
the success of those light rods. If I were to use a plug for the
top ferrule it would be quite fragile since the plug would have
to be extremely small to fit inside the blank at that point. That
is just another detail in my six piece rods that empahsizes the
care and study that has gone into their design.