The SG has been in continuous mass production since 1961. The LP was produced in limited quantities in the '50s (guitar music wasn't nearly as big until The Beatles hit in '63/'64), while the SG was produced in great numbers, in various forms - special, standard, custom - and was cheaper than the Les Paul when the LP went back into production in 1968 - as it still is today.
'Course a '64 SG standard is a little different from the ones produced today - most notably because today's Standards have a 2 piece body, but also the shape has changed slightly. A Custom Shop re-issue is 1 piece, curvier and has more pronounced bevels around the edges than a Standard.
