I have noted from Spookshow's loco encyclopedia and my own investigation, that the Atlas Classic U25B shell has these "tabs" that fit into the "notches" in the mechanism.

Thus when removing shell, you should *very carefully* use the "toothpick technique"
to spread the Atlas Classic shell (and I suspect the Atlas/Kato shell), to avoid breaking the tabs or damaging the shell when removing it.

When you are doing that, be very careful about not bending the Atlas Classic long brass wipers that go from the mechanism to the trucks (don't ask how I know that):