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[thump_rrr]

What stock is this?

That rifle is a CZ 457 in 22LR. With that stock it is called the Varmint MTR model.
In Canada it's available in 16", 20", and 24".
They are threaded in 1/2" x 20 UNF.
I believe that CZ USA only imports a 20.5" version without a threaded barrel.