The early Tk26 carburetors, which are found in the U.S. on the 1982 and 1983 Kz550H1 and H2 Gpz550 models, are slightly different than the later, simplified Tk26 carburetors. The early carbs have an additional, "intermediate" main jet circuit, and this system required a plug over the pilot jet.

This photo shows a replacement plug rather than the original factory plug.

There is a passage between the intermediate gallery and the pilot gallery.

This passage is approximately 6 mm away from the mouth of the pilot passage. The later Tk26 carbs and the Tk27 carbs did not have the intermediate circuit so they did not have the passage between galleries, and did not use a plug over the pilot gallery. To compensate, the later carbs used different main jets, needles, needle jets, and main bleed pipes.

This is what the OEM pilot plug looks like.

The original factory plug had a spike-shaped appendage protruding from the end of the plug toward the pilot jet. It was a small enough diameter that fuel could flow around it to get to the pilot jet. But it was substantial enough to occupy some of the volume in the pilot gallery. It is unknown if this displacement of volume in the gallery affected operation.

This drawing describes one example of the OEM pilot plug.

This drawing describes a replacement to simply seal off the mouth of the pilot gallery.
This shows the dimensions of the replacement plug.

The o-ring is being made to seal on the top and bottom of the groove rather than on the inside diameter. The tope and bottom tend to be smooth whereas the inside diamter of the plug groove tends to be rough from the groove (parting) tool chatter.

Here are some of the early attempts made out of acetal (Delrin).
(Not "slippery acetal" which may contain silicone)

Material:
McMaster-Carr part# 8497K151
5/16" Acetal (aka Delrin) Rod. 4 feet.
Brand "Quadran" 021001280172
"Acetron" GP Rod Nat
.312" x 48" measures 0.3150" to .3160" OD
(The rod is not perfectly round.)

As a test, it was immersed in automotive gasoline (10% ethanol) for a month.
After a month, it measured 0.3150" to .3160", same as before it was immersed.

They have been installed and seem to work well.

The plug is held in by a peg on the inside of the float bowl. The clearance between the end of the peg to the mouth of the pilot gallery is about 2.2mm to about 2.7mm. To be safe, the thickness of the head of the plug should be less than 2.2mm.