Inside or outside the quotation mark?

[forinfo]

Hello,

I wonder if the period should go inside or outside the quotation mark in a sentence that ends with a single letter or a number.

I know when it comes to punctuation of quotation marks, the American and British ways may differ.
(My understanding is that in American style periods and commas go inside the quotation mark.)
Are those two different in these cases also?

Ex) He italicized the alphabet "K"
I underlined the number "10"

Thank you for your help.

[bhaisahab]

Ex) He italicized the alphabet "K"
I underlined the number "10"

In both of these examples the full stop should come after the quotation marks because they are included in the sentence.