Are you sure the nosecap is pewter? The P53 Enfield had brass furniture.
The P56 Short Rifle (also P60 and P61) 2-bander had iron furniture.
25 was the bore gauge marking usually used to indicate .577 caliber (although true .577 is actually between 24 and 25 gauge). It seems that some Birmingham manufacturers made their barrels closer to .58 caliber (presumably for sale to Northern purchasers), and these were marked 24 gauge.
Geoff Walden