I actually own a super 20 silversonic alto that has a higher serial# than that and it plays beautifully. Considering it has 90% original lacquer and the key guard needing to be resoldered is the only problem with it, I would say it should definitely be worth the money. Are there any major dings on the horn? If you have some pictures of it that you could post on here that would be a lot of help.

 

No dings at all. I don't have pictures. The horn is at a local music store.

 

There's a later model Silversonic on e-bay right now with a few bids on it. It's around $1,700. It does look like it's in mint shape though. Based on that, it sounds like a OK deal. You'll probably get your money back if your patient, or you may find a real good horn and keep it.
Let us know what happens.