In his Darstellung des Fabrik und Gewerbswesens in seinem gegenwärtigen Zustande…, Stephan von Keess states that horn players needed three horns to play in all keys, and that those horns were somewhat smaller1. An information supported by Richard Maunder in his article Viennese wind-instrument makers, 1700-1800 : in Mozart’s opera Le nozze di Figaro, one can find quick changes of key that require an incredibly fast change of crooks from the horn players ; Maunder suggests that they would not switch crooks at the time, but instruments instead2.
Furthermore, in the same article Maunder writes a few words about Starzer’s death and the sell of his effects, which shows that fixed-pitch horns had still a great importance at the time :
« The announcement of the sale of Starzer's effects in 1790 shows that,
up to his death the previous year, he was still manufacturing horns in a
variety of fixed pitches as well as Inventions horns with changeable crooks3. »
The instruments sold are then listed by Maunder : 36 fixed-pitch horns (plus 4 hunting horns), and only 6 Invention-Horns4. The difference in numbers is astonishing. Obviously fixed-pitch horns were not just « still around », they were the most common thing one could find in a horn-manufacturer’s shop like Starzer’s in 1790. This is one year before Mozart’s death, and almost 40 years after the Invention-Horn’s appearance in Dresden.
Finally, when searching for historical XVIIIth century viennese horns, I encountered many master-crooked horns which, originally, were actual fixed-pitch horns transformed at some point in their existence to receive crooks, by cutting the lead-pipe and transform it into a socket. As an exemple, I found several historical Kerner horns (there is no surviving Starzer horn5). Those horns were all master-crooked horns, but all without exception started their existence as fixed-pitch horns ; some of them still are. I found only one Kerner Invention-Horn6.
Again, even in urtext-instruments, fixed-pitch horns are quite dominant ; but those massive transformations also demonstrate the need for crooks at the time. To conclude, fixed-pitch horns and « transitional » master-crooked horns are strong leads.