baretta s55 o/u

whats it worth? i have a baretta s55 o/u ,double triggers,28 inch,excl cond. most people seem to be familiar w a s55b. but mine appears to be a s55, not a (b). did they make both models? gun value books refer to a s55b w selective trigger excl cond around 500.00 i cant find a plain s55 no where. made in 1964 (xx-7). any help on clearing this up for me appreciated!