Every 8-12 years the USPOD published a collective "Postal Laws and Regulations" volume. These included 1879 and 1887. The 1887 volume was a complete reorganization and renumbering reflecting a lot of changes.
From the 1879 volume, section 294 describes the procedure after the mail has left the originating post office:
The 1879 procedure clearly had been replaced by 1885 by some other less cumbersome method. Here is the 1887 PL&V text in section 533, which specifically outlines the telegraph procedure to be used by the postmaster:
I am unsure exactly when the procedural change was made between 1879 and 1887, but apparently before this letter was sent in 1885, but the answer is likely in the US Revised Statutes volumes.
Add: Skipping forward many years to the PL&R compilation of 1948, it is still on the books: