The rifle was originally designed (as the Pattern 1913) for the .276 Enfield, not the .303 British. The .276 Enfield has a very large diameter case (.528") so the space required for 5 rounds of .276 was large enough for 5 rounds of .303 or 6 rounds of .30-'06. There were no "5 shot" Model 1917's; all will take six rounds. But the ammunition was issued in 5 round M1903 clips, so in practice the rifle was 5 shot and was often described as such.
Later, some sporterized Model 1917's and the Remington Model 30, which was made from surplus M1917 parts, were made with a true 5-round magazine.
Jim