Its not a strange serial number prefix. You have to "read between the lines" so to speak with those newer serial numbers (early '80s on). The SCSW only has prefixes they've confirmed. So look for ones before and after yours to get an idea of when yours was made. Or you can call S&W Monday and ask them. They'll tell the full date also.
Nice find btw, I've had two of those and like an idiot got rid of them. I'll also second that the wider front sight and heat treating for +P make it the pinnacle of model 60 production.