Well, a lot of old Sunn gear didn't actually have serial numbers. I believe the 412s (is that the actual model number, or are you just pluralizing 4x12?) was at earliest 1970. The one introduced that year was a guitar model and was refered to as the 412s, and had Sunn Transducers. They were available until at least 1975, and were definitely gone from the Sunn roster by '79. Your best bet at dating it is if it still has the original speakers, checking the numbers on those.
Here's The site for Sunn information: http://sunnforum.ampage.org/