Denny imported stocks for both the Brno M1 and CZ 452.
They're so close its tough to say. I know this because he accidentally sent me a M1 for my 452. Of course he offered to send the correct one, but all that was involved was minor tweaking mostly at the front of the receiver.
Seems to me it was a slight length and radius difference.
That said, and if I'm right, it looks like someone got pretty ham fisted on the mill cutting the left side of the receiver inlet.