I have exactly zero documentation for the information, but I was told years ago, when I collected .303 for awhile (I always loved the SmlE rifles and the Bren gun), the the headstamp you show was made for the Republic of Ireland.
They had, of course, primarily British weapons, including many hundreds of post-war manufacture Rifles No. IV, Mark 2. In fact may years later on, perhaps 16 or 17 years ago, a lot of those No IVs were imported into America, so brand new in original packaging. We had about 100 of them at our store - several groups of four and five consecutive serial numbers. I bought one of them, although as a shooter of Enfields, not a collector, I unwrapped it, saving only the package label, and have shot it a couple of times over the years since the store closed down.
Those rifles, and the English label on your box, lend credence to the “Irish connection,” but again, this is all hearsay. No documentation at hand.