Hello timx186,
Congratulations! Looks like a nice one. Don't know if you are aware of this, but the X-375 model designation was used for a blonde-finished instrument. The same instrument in sunburst would have been called an X-350. If the guitar has the X-375 designation on the label, it was most likely refinished at some point.
The sunburst doesn't look like a '57 sunburst to begin. So assuming that the work was done at Guild and by looking at the style of sunburst, I would say it was probably done during the '60s. However I should add that it's hard for me to tell from a photo if we're looking at a factory finish.
Sincerely,
Hans Moust
www.guitarsgalore.nl