If it's 1/10" wide from top to bottom and pinned in place, most likely the Patridge sight was just reshaped by rounding it off by a previous owner.
Or it could have been changed to round; look for punch marks around the pin and flattened pin ends. They should be nicely domed if original.
Or special ordered that way. You'll need a letter of authenticity to verify if it is a special order factory installation. If a letter doesn't confirm it, someone else did it.
Either way, it's easily replaced with the proper Patridge blade.
And lastly, if the gun has the 1/2 round, fairy thin tapered front sight and it's NOT pinned in place but integral like a fixed sighted gun, the barrel is from a non-target .22 (very rare Coast Guard issue), and the barrel serial # will be in the #650929-650953 range and not match the gun's #. (If the gun was in that # range, I'd say someone added the rear sight to it, but that's not the case with your serial # 64XXXX.)
That vintage K22 should have a rear sight with one elevation screw for adjusting but no 2nd locking screw.
The Call flush bead Patridge front sight blade was only standard on the early versions, but optional later in the serial # range when plain Patridge front blades are the norm.
Is there a star following the butt serial # or a date stamp under the left grip on the grip frame near the 'toe'? This would indicate a return to the factory to rework the sight, refinish the gun, etc.