Reading the discussion so far, i have noticed that "660W 600V" is a common rating i see on all types of American sockets that made their way here. If they are rated for such voltages i assume there is atleast one application where they are legit to use at such voltage ?

It is similar like a lot of sockets intended for 230V are rated as "250V" or even "500V". It is the rating the socket itself is designed for by itself, not that much related to what lamps are or are not made.

For incandescents I'm surprised anything above about 250V  even exist, given the huge problem even a slightly higher voltage means.

Here on old tram switches were usesing about 4 230V incandescents in series as "switch locked" indicators (activates once the tram enters an area where the "feeler" wire on the patograph touched the switch control feeler and so it have send signal to switch to one or other direction) - the whole switch system was directly supplied from the 600VDC traction power, including the indicator lamps (at one place they were in the form of 4 R7s reflectors arranged to directly illuminate the switch section of the tracks, so the driver can visually check the condition).
Today they were replaced with different system and LED indicators