All of my plates have been on the right hand side of the transom up towards the top of the transom...I am not familiar with your particular model, but it seems to me, this is suppose to be a pretty standard location...
Well, a lot can happen in 31 years, so I'm not surprised the placard is missing. While there is a formula that can be used to determine passenger capacity, it is only supposed to be used for homebuilt boats. Fortunately, Sea Ray is still around, so check with their customer service department. There is also a Sea Ray subforum here at iboats.