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careful running slugs with it then. Turkey chokes are fuller than full all things equal.
should be some markings on it and probably best to measure the bore like you are going to do. In saying that...

muzza you are probably correct but they did do a 21" barrel so could be ok

Found a few things about the wingmasters. This sort of thing popped up several times but while a copy of someone elses correspondence, it does come from Remington.
about half way down the page it...

x2 on what Husky1600 said.
I always hankered for a short barrel for my old A5. Good for puke shoots in the scrub and rabbits in tight stuff. Never happened.
if you are shooting at quick moving...

Maybe skeet? what choke is it?
Possibly made for buckshot/slugs for deer etc
If the stock is quite short it could be a junior/youth model.