The wire loop is only there as a tie down point for the canvas breech cover that became standard issue around WW2 to prevent mud or dirt from getting into the action.
When the canvas cover first came out they tied it to the parade swivel mount that was in that position on earlier model SMLE rifles. When they simplified the design they eliminated that swivel mounting but still needed some place to tie down the breech cover.
Heres what the action covers look like.https://www.amazon.com/British-Enfield- ... B00JHLZPVC
Garra is correct about the linkage securing the magazine of the earlier Lee Metford and some Lee Enfield rifles. This was no longer an issue by WW1 and WW2.