The electro pencil marking is not unusual. The serial number will be on the back of the bolt handle and on the receiver ring. If these have been polished up (linished in British ordnance terminology), then the number would be repeated on the sidewall. That sight is the more desireable adjustable model, and not the as issued two hole flip version.
In my understanding of British No.4 rifle production, the exception is the rule. To repeat an old expression from the Jouster dot com forum, 'never say never when talking about Enfields'. The Brits found themselves in 1939 with insufficient production capacity for the tools and equipment of mass warfare. They shopped abroad as much as they could afford, which soon became a favourable credit program called Lend Lease. In return the US and Canada had inspectors at the UK plants making damn sure they were using everything they had to the maximum efficiency. All raw materials were centrally controlled, which led to the famous pictures of scrap metal drives for aluminum and steel. Little shops that made table legs were ordered, not asked, to make rifle stocks. Plants with machine tools were given drawings, materials, jigs and gauges, and told when to expect the inspectors' visit. So, as long as parts met the specifications they were shipped to central plants for just-in-time assembly. The less of their precious capacity was concentrated for the Germans to bomb, the better. After the war and everyone took a deep breath, the companies could afford to look back at their production and give it the extra attention it needed to be truly presentable.
I suspect the rifle at question was sent for Factory Thorough Repair or FTR, and that is when the electro pencil engraving was done. It just never got the FTR markings.
