The .276 Enfield cartridge didn't last very long and was not formally adopted by the British due to the onset of hostilities in August 1914. There was a small number of rifles chambered for the .276 cartridge manufactured for Army field trials, called the P-13. After the decision was made to stop further work on the .276 development, the P-13 rifle design was then modified to accept the standard .303 British cartridge and thereafter was called the P-14, which was manufactured mainly in the USA, and in large numbers, for the British military. I don't know about the P-14's magazine capacity situation. The .276 Enfield cartridge case was somewhat fatter (larger diameter) than the .303 (or for that matter, even the U. S. .30-'06) case so the P-13 might have required a deeper and wider magazine well in order to hold 5 of the .276 cartridges. When the P-13 was converted to use the .303 cartridge and became the P-14 rifle, there were probably no changes made to the original P-13 magazine dimensions. As the P-17 (M1917 Enfield) was pretty much the same as the P-14 except for the cartridge and likely used the same magazine box dimensions as the P-13, that's probably why it could hold six .30-'06 cartridges. This is all guesswork as I have no P-13 or P-14 rifles to look at and measure.
I wouldn't go as far as saying that the P-13, P-14, and P-17 actions were Mausers, but they operated much the same. The U. S. '03 Springfield rifle definitely used a fairly close copy of the Mauser 98 action, and it did infringe on the Mauser patents. I have not heard of any Mauser patent infringement issues regarding the P-14 Enfield action.
