Hard to know after all these years, Nash B wrote about Charles Wickes working at A&F with Capt. Harold Money in the gun sales area. I can't answer the Hemingway/Parker scenario, I agree with Austin that possibly a Parker was the only shotgun available when the foto was taken. Hemingway mentioned a Model 12 in his "True At First Light"-a post mortem rehash of the second African Safari he (and Mary Welsh H.) took with Phillip Percival- and annotated with a forward from son Patrick Hemingway.

He may also have used a Browning 12 Over-Under and some European/English shotguns, but I have never read of his owning a shooting a Parker-- at least to date.