Mike, it's probably a 90. Frank Berto gives some measurements in his 1990 book "Upgrading your Bicycle."
B17: length 11.1 in, max width 6.7 in, width at half-length 3.2 in
Professional: length 10.7 in, max width 6.4 in, width at half-length 2.6 in
Ideale 92: length 10.8 in, max width 6.8 in, width at half-length 3.1 in
Ideale 90: length 10.3 in, max width 6.1 in, width at half-length 2.5 in
The Ideale 92 is very similar in basic shape to a B17, and the Ideale 90 is similar to a Professional. There might be a sport saddle that is closer to a Pro than the 90 is, such as perhaps the 80, but I have no data on anything other than the 90 and 92.
I have a natural Rebour Ideale 92 in better condition than nearly anything out there today, with chrome frame and rails and loads of screw travel. Looks like I should BIN it around $600 with a starting point of $200.
