Ok, I'm retarded tonight - went and took pictures and all then realized that you weren't interested in the Parkway model...
...maybe shove some pillows there until the little one doesn't need them any more and it'll give me the excuse to post the pics?
What I did notice is this and I'm not sure what measurements may help you - but it's really the shape of the base the suits the 993 rear seats so well that is the difference with other car seats. Other car seats can have quite a rectangular base - even if they are seemingly narrow they will fit the front of the rear 993 seat, but not the back.
So even if I could post some pics I wouldn't know how to show you relative measurements of the curvature of the part that fits in the back part of the seat. To wit, if you look at the Britax web site (
http://www.britax.com) you'll find specifications there, but it's of the widest part of the base and doesn't describe the narrowest (because it's rounded).