There is no info available via the serial number as far as I know. If you post some pics, I'm sure that someone here can at least narrow the year down, and probably give you a lot more info about your Sonic.
If you want to sell it and have a figure that you would take for it, I'd post it (with pics) in the classified section here; Or to find out what somebody will pay for it just list it on eBay.
Old Sunns seem to be bringing top dollar lately.
