It's laying on its face.
There may be a builder's plate just inside the side flap door which would list the heat value.
My best guess is 20,000BTU
Casey

Another picture anyone . Thanks sombreuil_mongrel for the tip.

It has a 4 " exhaust (chimney) if you look at the top picture It shows. under the exhaust whole it has a complete opening to draft cold air in with the exhaust. the area of the opening is about the same size as the 4" exhaust if not bigger. Will I have a problem when I first light it and the fumes just coming out into my work space. I hope you understand my point.
I will assume it just take the rout of least resistance once the chimney gets hot and it will go up instead of down to the opening

I had a very similar heater for many years (it looked more like an old jukebox with a big chrome grill) it measured only slightly larger than yours, and was 18,000 BTU according to the builders plate. It had a 4" outlet, but came from the factory as either vented or _unvented_. Up to a certain BTU capacity, not venting a gas heater may be permissible. There are unvented bathroom wall heaters still for sale.
Casey

Get a pro to check it out before even installing it. It's not something to bet your life on without that.

I have a very similar but larger version of this heater in my cottage. Mine is ugly as sin. when I bought it I thought
it would be the first thing I'd need to replace but as ugly as it is and as big as it is, it works so well that it's still taking up space in my living room.

Good luck with your heater. I haven't had any problem getting service or parts for mine.

Susan