It's printed from a Series 1934 face plate but a Series 1934A back plate. (You can tell because the back plate number is in a larger font size than the face plate number.)
No, just economizing--there's no reason to throw away still-usable plates, and no reason to go to the trouble of ensuring that old backs are never paired with new faces, or vice versa. There were a *lot* of mules printed in the 1930s and 1940s; in some series and denominations, the non-mules are actually the rare ones.
