Is your Sonic maybe an l-40? That was a 60 watt amp, but was the head itself was actually marked as an "l-40" or just as a "Sonic l", since the "40" part of "Sonic l-40" was probably simply a reference to the upgraded D140F loaded cabinet?
Who knows? Obviously Mr. Sundholm would be the one to ask, it seems anomalies abound back in the Sunn heyday.
I've got a clearly marked 2000S with an also clearly marked inspection date of 4/72, and according to everything I've seen or have (including a 1972 catalog), the 2000S was renamed "350B" by 1972.