Thanks for your insight, Widget.
Fortunately, the newer SG520 has been tested by a vintage audio technician with no issues at all.
Functionally, both preamps work just fine, with all the source buttons as well as bass and treble buttons in proper working condition.
As you pointed out, these vintage gear, produced in the mid-1960s, might need a thorough check-up, just like used cars.
Still, they sound wonderful, given their production year!!!
When I got this result, I just thought to myself maybe this newer SG520 might need some more listening time for a fuller/mellow reproduction just like its older brother (their serial numbers are two hundreds apart). Just like you need to drive your new car for some time for full, efficient driving experience or if you are a runner, you need to wear your new running shoes at least a couple of weeks to break in...