Thank you for the letter scan. You may or may not know that San Francisco had a large French immigrant population since the 1850s. Also, Otard Dupuy & Co. still exist as cognac makers under Martini & Rossi.
How do I recognize that this cover went by rail and stage? Is it only the dates of the postmarks on the letter??
There are no markings/would be no markings to show transit. Since you have disproven ship mail, it is the dates that show the overland route.
And is then the time for the trip of ca. 17 days correct?
See source quoted in my first post above. By date of mailing, there was only about a 900 mile gap not covered by rail in a approximately 3000 mile trip from San Francisco to New York.
Possibly in print sources. But the distance between railheads was always changing - how accurate could a schedule be?
Is this website, linked by hy-brasil, the only source reference for this?
If you don't believe what the USPS site says, not to mention Richard Frajola, would any other references be convincing? In any case, there should be print references with details.
You demand further proof, yet you cite an unnamed source from memory. Unless you have eidetic memory (yes?) I would not trust memory so absolutely.