2.1 x 5.5mm Power Cable Fits Elecraft, Chinese HTs, some MFJ, others.

2.5 x 5.5mm coaxial power plug with 6 feet of #24 wire.
This one is commonly used on Most Chinese HTs, Elecraft, Older ICOM HTs like the IC-02 series, and some MFJ equipment.

Here's a trick for determining what size plug you have.
You may not have a caliper or micrometer, but you probably have a standard set of drill bits.
5.5mm is very, very close to a 7/32" bit. And a 3/32" drill bit (or audio plug) will slip inside the center barrel of a 2.5mm plug, but not a 2.1mm plug.

Similarly, a 4mm OD connector is very, very close to a 5/32" bit.
And a 3.5mm connector is just a tad smaller than a 1/8" bit.