1957 or '58 is gonna be real close. I don't believe I have seen that model before. The "stereo" position on the function switch sways it toward 1958, but there was a stereo tape deck in the 1957 300H Concert Grands as well. Tube codes and speaker codes may be needed to get closer.
As to the survey of how to date a Magnavox, I vote "none of the above." My vote would be for build date, rather than sale date or date a model was in the catalog. Magnavox mixed and matched components that were not in the catalog. Magnavox was typically sold in music stores or furniture stores, so may have sat on the floor into the next model year. Magnavox purposely made its products difficult to date, perhaps so they could set in a furniture store and not appear to be obsolete before it got sold. I would rather refer to an instrument as a "43rd week of 1959" and let everybody come to their own conclusions. Whether that would be a late 1959 or an early 1960 is a matter of conjecture. The fact that we know it still exists and that someone is restoring and enjoying it 50+ years after it was built is good enough for me. Perhaps a listing of all the instruments owned by members of this forum, along with tube, speaker and turntable codes would be useful to try and figure out what Magnavox was thinking. Include as well the models numbers of the amp, tuner, MPX adapter and changer. Do that for tube and solid state models. Include any model nomenclature, such as Concert Grand or whatever. The tractor guys do that and it is helpful if you get an item and are trying to figure out what you have.