Looks like a rear sight from a Ross MkIII. That is a surprise to me.
The HV, SC and, most importantly, "DP" (Drill purpose) stamp are telling, as they denote a .303 barrel at the end of its useful life.
Lots of good information about the rimfire conversions, and the Cooey rear sight affixed to your rifle on this site out of the UK - http://www.rifleman.org.uk/Enfield_Rifle_No.2_and_.22RF_Mk.III.html.
Looking forward to seeing more photos.




























