Very cool looking antique pool table. Thanks for sharing.
Unfortunately I don't have much info for you, but hopefully some of this may help.
The National Fixture Co. of Cleveland Ohio was located at 24-26 Frankfort St., and later 631-703 Frankfort St (probably the same physical location with an upgraded street numbering scheme.
Also, there's a good chance the National Fixture Co. might have been a retailer/distributor of pool tables from other firms, and simply affixed their labels to the pool tables they sold. You'll see from a few of their company listings below, they're listed in a separate category from proper pool table manufacturers. For example, this entry from the 1911 Cleveland City Directory:
However, in the 1922 Cleveland City Directory, they took out an ad listing themselves as "Manufacturers of Billiard Tables":
They also had a branch in Chicago, IL, at 189-193 Iowa St. as seen in a 1909 liquor dealers trade magazine:
I also came across another name plate from National Fixture Co. mentioning "Master Cushions" (which yours does not say):
I checked with one St. Cloud, MN antique pool table dealer, 'Antique Billiard Tables and Accessories' and they've not hear of them. You can also try reaching out to Ken Hash at Classic Billiards. He's always willing to help and knows a great deal about the old pool table makers.
For further digging, I'll refer you to two experts. You should reach out to them, in this order:
Ken Hash at Classic Billiards in Fallston, MD. He's the guy to talk to for this type of stuff. He is always happy to at least have a look, and if he can he'll give free advice etc.
If Ken can't help you, try Joe Newell out of Clay Center, KS. He runs Newell's Antique Billiard Supply, and he is the foremost Brunswick expert in the US.
It's a great antique pool table you have there, whatever it is. I know this doesn't answer your question, but hopefully it's a point in the right direction for further research.
Good luck and let us know what you find out.