any ideas on who made this chair?  

From back here looks like Elm!

More photos would be great, of the underside too!

Any marks at all?

A photo of the side of the chair would help a lot but please get down at the level of the chair if you can. There's a lot of distortion when the camera is not on the same level as the subject matter.

The top of the back looks very familiar to me, and if it's familiar to me then someone else will probably know it for sure. (I'm not that great at IDs despite having seen a billion photos and chairs in person by now.)

American, I would say. Perhaps Viko Baumritter, maybe?

Yes, Baumritter--found one on etsy that is labeled and marked. I redid a pair of these a long time ago but they hadn't been refinished like this one. You can see the dull, dark original stain on the front rail under the seat frame on yours.

Maybe elm, maybe ash?

https://www.etsy.com/listing/184887037/antique-danish-signed-baumritter-...? (ignore what the seller says about era and style)