The projectile on a 30-06 is .004" less diameter than that of a .303, so I'm not sure that it would be much use as a wear indicator.
No 4 buttplates are often mazakalloy (dull silvery gunmetal colour), sometimes black steel and sometimes brass, particularly on the newer Mk2 models.
The SMLE and No 4 rifles are both 44.5 inches long with standard length butts, so shorter than an M-17.
7.62 x 51 are the Indian made 2A or 2A1 version of the SMLE; sniper/target versions of the No 4 with heavier barrels (L42A1, Enforcer, Envoy, L39A1) and standard weight barrel conversions (L8 series,Sterling conversions, DCRA types, sundry other gunsmith conversions)
The No 4 did not use the long sword-type bayoet