SMLE No.1 Mk1 Sight insert when .297/.230 Morris Tube fitted

I picked up this little spring steel sight attachment which is marked M 4 .297/.230
I understand that the M 4 indicates Morris Tubes designed to fit .303 barrels. The next question is which rifle this was designed for? It wouldn't fit a Martini-Enfield sight because the sight leaf is too thin; the same problem with the Lee Metford or Lee Enfield rifles and carbines. However, it does fit the longitudinal groove on the SMLE No.1 Mk1 sight leaf. It is not a brilliant fit and needs the slide to squeeze the attachment against the rear otherwise it is too loose. Fitting it in this manner would then prevent the slide being moved forward as the sight attachment would then become too loose. Hopefully some of the experts on here will come up with an alternative suggestion as to which .303 rifle this was designed to fit. Included in the pictures is a .297/.230 Morris Tube round alongside a normal .22LR.