I believe my W 90's are 1964 models, and only the unvented rear panel is sand filled. There are two rear panels that cover the devided cabinets, and the right side (when looking at the backs) is the solid panel that is route and sand filled whith a thin panel over the route to contain the sand, and the left side cover is vented with slots going side to side from top to bottom. There is no sand filled panels on the outer walls of the cabinets or the divider panel. I did change out the Motarola style input jacks and installed Banana 24K plated jacks for easy of hookup and swapping speakers. These speakers sound wonderful compared to my W 70's, and my only concern is that my 60 watt per channel Dynaco Mark III amps may be too powerful for the W 90 and the W 70 speakers. I'm keeping the volume below 4 to play it safe!