This came in a very old fly bag. Do you know what it is? I have a few ideas, but I am interested if someone knows for sure what it is or who made it

Looks like a reel pouch. Mounts on the belt and the reel sits inside to hold the rig while you dress your fly, retie your tippet, or irrigate the stream.

Thanks for the response. I thought it might be a reel holder also, but it isn’t even wide enough to hold an extra spool.

Is it a rigid material?

To me, it looks like some sort of holster. A net, or fly box or something will have a protruding disc that will fit in there as it hangs on your belt.

I looked at that aspect too. Remember, back in the day fly reels were all narrow and had small arbors. But I'm just guessing here.

It is a hard plastic rigid material.

I'm guessing a round fly box holder. I've got a couple of the circular fly boxes from years back, and surprisingly they hold a large number of flies.
Probably attached to the bib of your chest waders and not on a belt.
As a reel holder it looks a little too narrow and rigid as you say.......But I've been wrong before!

IT'S a UFO! Unidentifiable Fishing Object. I was looking at the round fly boxes in your other pic, and wondered if it was for them, but it's way bigger. ?? IslandJim

That is the belt holster for a collapsible net.

Thanks for the reply. That looks like it. I thought about a net, but there was no hole in the bottom for a handle. A collapsible makes sense. It is also the right size for the old fly line spools.