The RIFLE date is on the Body (what the Americans call the 'receiver) of the rifle and it is 1917. That is the year that it was built as a RIFLE.
Following the First World War, it was rebuilt, at Enfield, and given a new barrel. The new barrel was installed in 1923, and that is where the "'23" on the chamber comes from.
It is a First World War rifle, rebuilt in the 1920s and put into stotage so as to be available a few years down the road, which is when that nice Mister Hitler began goosing and stepping about the landscape. Our politicians woke up (about 6 years later) and started rearming. Fortunately, this rifle was available, having already been rebuilt.
There is a LOT of information in proof marks, ESPECIALLY British ones, once you learn to read them.
Have fun!