Remington Model 31 riot...or not? Or what?

[Anzac15]

Found this today at the local fun store.
Remington 31, uh, riot gun? Receiver has serial number 127957 (1947-1948?), but barrel is mismatched. It is numbered 57516 (riot gun serial number?) with anchor marking and CL. It is NOT marked 'CYL' or 'FULL' anywhere. Barrel is 20 inches.
Patina is matching all over, shotgun has been in this configuration for a looong time.
So, what do we have here? Something the military put together? A police department shotgun? Or somebody somewhere a long time ago got hold of a riot barrel and fitted it?
Either way I figured $150 was cheap enough.

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