During the era in question, the US industry standards, according to most published reports, were essentially in-house duplication of the Birmingham Proof House Rules.
According to "Proof Tests and Proof Marks", Part IV: Practices of Manufacturers in the United States" by LTC Calvin Goddard, and published in the May-June 1934 issue of Army Ordnance magazine, Winchester stated: "... the standard laid down for the British proof regulations adopted by the Guardians of the Birmingham Proof House dated 1925 is followed".
The 1934 quote doesnt address your 1887 question directly, but in absence of information to the contrary, it is reasonable to conclude that it was merely a continuation of previous practice. In 1887, Winchester was likely following the Birmingham Rules of 1887, or a close approximation. They would have had to do so, at least as a performance standard, or Winchester arms proofed in England would possibly fail proof, which would be, of course, an embarassment to Winchester.
Good luck
Regards
GKT
