Yes. I got bugged by the gender thing and went looking it up, and it turns out the explanation I thought I had learned was wrong. It's not the word "his" getting abbreviated, it's a genitive -es ending that we no longer use. The apostrophe eliminates the -e-.
And then in plurals, theoretically you could still have -'s on the end of a word, but since it already ends in -s it gets swallowed up, so the apostrophe goes at the end with nothing after it.
Thus: Those are John's bass guitars' strings. Originally one might have written "guitarses," but you theoretically elide the -e to write "guitars's," but two -s sounds are the same as one so you just write "guitars' " instead.
