This coin is the scarce '42 plain threepence, all right!
In almost all cases, the image of George V1 is lightly struck and the lettering around the image is well stuck. The coin pictured here is very typical in this respect. Perhaps this is the reason why the '42 Plain is so scarce.
This coin was clipped AFTER it was struck.
Reasons:
1.) The raised lip on the obverse side around the edge of the clip itself is evidence of that. The direction of shear was from the reverse to the obverse. The raised lip on the edge of the shear itself would have been flattened during the striking process, if it had been a blank or planchet shear. That is not the case here.
2.) I see no evidence of the Bleakesley effect on the opposite edge of the shear. With a shear this big, a Bleakesley effect should be quite pronounced.