Punctuation of eye dialect

Suppose you have a sentence which concludes with the eye dialect somethin' for the word something. Where does the period go? Which is correct?

This is really somethin'.

This is really somethin.'

It seems to me that the apostrophe is part of the word, and hence attaches to it as opposed to the sentence. Thus, I'd put the period after the apostrophe. The second construction looks like an error, with mismatched quotes.

You're correct—the apostrophe is there to signify that a letter (or letters) were omitted from the word, and so, the apostrophe goes with the word and before the period.

While periods go inside quotes in American English, this isn't a quote.

Place the apostrophe before the period in this case.