It has been a challenge to document the Prix winners.

This is not quite correct
“Manly Sport of Pigeon Shooting”
The Illustrated American March 24, 1894
http://books.google.com/books?id=tHVNAAAAYAAJ&pg=PA309&dq
In the first year of the competition, 1872, this was won by Mr. George Lorillard, of New York. For the next six years Englishmen took it, and then, in 1880, Count Esterhazy won it for Austria. Italians have won it six times, Count Cuidicini carrying it off three times. This year it was won by Count Lichy, an Austrian.

The problem is that there were 2 grand prizes; the Gran Prix du Casino and the Prix de Monte Carlo AND Prix d’Ouverture and (later) Grand Poule D’Essai

This is the best source that I've found. Sporting Life didn't report the Monte Carlo competitions regularly until the 1890s
All the Gran Prix competitions were in January
https://books.google.com/books?id=Vmg9AAAAYAAJ&pg=PA62&lpg

Gran Prix du Casino
1882 – George Lorillard, New York
1883 – H.J. Roberts
1884 – H.R.H. Comte de Montecupo
1885 – M. De Dorlodot
1886 – S. Guidicini
1887 – Comte Salina
1888 – not recorded
1889 – V. Dicks
1890 – S. Guidicini
1891 – Count Luca Cajoli
1892 – Count Trauttsmandorff

Prix de Monte Carlo
1883 – S. Guidicini
1884 – M. Drevon
1885 – Capt. Shelley
1886
1887 – Seaton
1889 – M. Guido
1890
1891
1892 – H.J. Roberts

Grand Poule D’Essai
1891 – B. Horton and V. Barker tied; C. Macalester (Philadelphia) 3rd
February 14, 1891 Sporting Life
https://digital.la84.org/digital/collection/p17103coll17/id/49722/rec/20

[Linked Image from photos.smugmug.com]

A thread about Charles Macalester
https://www.trapshooters.com/threads/charles-macalester.843725/

Sporting Life April 29, 1883
Philadelphia vs. England
https://digital.la84.org/digital/collection/p17103coll17/id/20466/rec/2
Charles Macalester is a gentleman of leisure, a Philadelphia born and bred, and belongs to one of the oldest and wealthiest families in the city. For the last two years he has lived at his country place at Townsend’s Inlet, N.J. He has been an enthusiastic member of the Riverton Club ever since its organization and is always selected as a member of all the crack teams of the club. He has been to Europe with his gun a number of times and his taken part in several shooting contests in Monaco, where a few years ago he was awarded the Grande Prix, a work of art valued at about $2000 and an equal sum in cash. ($2000 then would be about $60,000 today)
In all his single and team matches, Mr. Macalester has shot under the name of Johnson, and it is, therefore, as Johnson that he has made all his records.



Another source
Baily's Magazine of Sports & Pastimes, March, 1902
https://books.google.com/books?id=xJAbAQAAIAAJ&pg=PA244&lpg