It retailed for $400 (typing that gave me an odd feeling — I'm used to putting "K" after prices these days). I think they made circuit changes, worthy of a new model designation or at least adding a "B" — but budgets were so tight they just put it into whatever cases they had around. Only when all the old cases were gone did they take a deep breath, and write a check for a new run, and then they could change the wording. Same with knobs — run out of the silver, go out and buy 100 black ones. Can't afford to bulk order 10,000. So the external appearance might be irrelevant to what's inside.
Yours has the Volume and Balance balance controls, so you're spared the annoying two-knob back-and-forth finger dance.
Everything is cheap except the sound, and that includes the pots, bottom-of-the-line carbon composite. I changed mine to metallized film (carbonized film is just as good) and the improvement was striking. As a friend put it (a sound engineer), "There was white noise before. Now it's just silence." That's with no music, just the sound of the preamp through the speakers. When playing music, the sound is silkier, less granular. You probably don't notice the granularity until you make the change. Then you smile.
The pot upgrade is pretty cheap (about $20 for both when I did it). You don't even have to bother with the Balance really, since it's always in the middle or close to.
