* There were hundreds of little shops in France that made saxophones. Selmer is probably the best known French saxophone manufacturer, but "best known" doesn't mean that you have a possibility that your horn was made by Selmer. I can guarantee your horn wasn't made by Selmer.
* "Amazing" and "astonishing" are synonyms. "Surprising" is close enough. As I said, I took Latin in school, not French. In any case, it's just a label.
I don't know why I didn't check Google or I did and I didn't post here. I will blame my muscle relaxers for back pain. Anyhow ...
Your horn is a Rene Guenot stencil. (Rare NSFW warning: some of the Guenot ads have topless women.)
Your horn has reduced key work: it's missing the chromatic F# key and doesn't look like it has an articulated G# mechanism. This description sounds like the "Model 0," after you bought some add-ons, or the "Simplimax" version of the horn.
I think that the serial number is based on the model of the horn, not a brand total. In other words, serial number 123 refers to the 123rd Model 1929, not the 123rd horn that Rene Guenot produced. That essentially means that I have no idea how old the horn is. Early as 1934 and as late as at least 1938, just based on catalogs. I don't know when Rene Guenot stopped producing horns. Start of WWII? Through WWII?
1. I read that these horns are high pitch. "High pitch" is a tuning standard. Modern or low pitch is A=440hz (some orchestras use A=442hz). High pitch is A=467hz. That means that your horn can't play in tune with other instruments and your horn can't be made to play in tune. (Note that some people do solder on an extension to the neck and fiddle with the tone holes. This can help, but you'll still have difficulties.) To check, you need someone to play next to you or an electronic tuner.
2. This is not a professional model horn and wasn't a professional model when it was released.
3. If the horn is high pitch, it's value is close to $0. If it's low pitch, maybe $100 to $200.
Oooh. That took 2 hours to write.