The skull's vertical measurement is less than 3/8" but its width is
more, so it can't get in.

I don't know what the skull's vertical measurement is but mice can
get through the tiny gap between skirting boards and floorboard in
our house.

I guess they just flatten their skulls. Think of a ball of plastercine
only a tad larger tha 3/8" dia, it won't go through the 3/8" piercing
no matter what you do but you can flatten it down to 1/8" thick and
put through an 1/8" high slot no bother.

Or perhaps some of our gaps are 3/8" !

Mary

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Cheers
Dave. Remove "spam" for valid email.