My answer: You have a "Model A" autoharp which were made up until the 1960s, but you have an earlier one that could go all the way back to the 1920s or so.
There's a later Model A on this page:http://stewartportguitars.com/pages/aut ... p-2-3.html
You can see that it's just a little smoother around the edges and has oblong keys instead of round ones. The "Model B" below is even smoother but has other design changes that are harder to see from the photos.
MOST autoharp restorers won't bother with a Model A since so many of them are falling apart, and the truth is they weren't all that well made in the first place. It wasn't until Chromaharp started making their version that Oscar Schmidt decided they'd better update their sound and their quality.
I have a type A that is even older than yours, and has a cracked face. Ironically it's still playable, so go figure.
If you're happy with a 12-chorder, you might talk to the fellows whose ad I just posted about getting his.
If you can find a 15-chord type A, the size should be about the same as yours. Another thing to consider is that Celtic music is usually played in keys like C, G, D, and A that are guitar and banjo-friedly. MOST autoharps don't handle those keys well. You might could look for a Chromaharp "Bluegrass" model or an Oscar Schmidt Appalachian model, which handle keys like D and A better.
There's more about those models here:https://creekdontrise.com/acoustic/auto ... unings.htm
I even have a Chromaharp Bluegrass model - I bought it when I was researching that article, but it needs new strings and a LOT of work.
I know that's complicated. In fact, my article about Making Your Autoharp Folk-friendly may not be all that clear to a non-musician, but you might take a look at it before you put a lot of money into rebuilding the harp you have.https://creekdontrise.com/acoustic/auto ... toharp.htm
Hope this helps,