How it cannot be a slide guitar I cannot see. That is to say, I think it's a Hawaiian guitar, meant for use with a metal "nut raiser" but playable as a regular guitar, too.

Why do I think so?

(1) it has the notes for open tunings on the paper guides on the fingerboard -- a "regular" guitar would have the standard EADGBE tuning noted;
(2) slack-key guitar was never popular enough on the mainland to cause anyone to build a guitar for the market (and there are several tunings for slack-key, too -- which one would you put on your fingerboard?);
(3) it has a paper guide on the fingerboard -- a fretted guitar with paper on the fingerboard soon loses the paper on the fingerboard, from finger wear -- the paper is not meant to be touched very often, so the guitar is meant to be played with a steel; and
(4) I tend to be pedantic, so if I say it's a Hawaiian guitar, that's what it is!

(Only the last point is important -- the others are simply logic)

Bob