Wow - that's worth 1 point if I'm satisfied it's right!
Perhaps you could post a picture of their emblem?
Quite difficult to find something appropriate online. The emblem consists of three parts: a left trapezoidal carrying the name ROCHET, a small heraldic middle carrying the interwoven R and S, and a right trapezoidal carrying the name SCHNEIDER. It seems that the post-WWII Rochet-Schneider with works front had the RS decal above the other two. But an independent coachbuilder as Grange Frères could have changed that. And I found a picture of a Rochet-Schneider Phoebus bus with a body by Besset where this was done just as on the puzzle bus. But unfortunately, this picture isn't online, just on good old paper in the book "Camions de France - Troisième Époque" by Jean Gabriel Jeudy. I attach some Rochet-Schneider pictures to show what I mean. Another indicator is the grill with many slim horizontal chrome bars and one vertical center chrome bar. Most Rochet-Schneider after WWII had this. And finally, Rochet-Schneider was located in Lyon, only about 75 km away from Valence, where Grange Frères resided. Now it's up to you to accept my "assumption with foundation", even if it no final proof.