So here are results of the next experiment.
After trying several times more, I chose next combination
:2402 dia, big (1.71") phase plug, remounted it several times
and got the same FR, within 1 db from each other.
Then I repeated it 4 times more, but with some variations.
1st variation (blue and gray lines): ...............
2nd variation (violet and green lines):
...........For the moment I am satisfied with the finding that EVERY 2404 could benefit
from placing a gasket between two parts of the horn.
Gasket should be 13 mm ID, 22 mm OD, and 0.7 mm thick.
PS to my post of 18 dec: I chose to have my PWT end tightly plugged.
It does not change results of measurements,
but prevents external noise to enter the tube during measurement and give wrong results.
(Don't ask how I.....)