The 'double-clamp' item is undoubtedly an Alfred J. Parker foresight adjuster for a Lee-Enfield. If the '03' is a date, then it'll presumably be MLE or CLLE models, otherwise and maybe more likely the much more common SMLE used in WW1 and WW2 until superseded by the No.4. It shifts the dovetailed block incorporating the foresight blade sideways rather than bending it.
To use it on the SMLE required removal of the rifle's nosecap in order for the adjusting screws to be able to get at the foresight in its dovetail. This was fiddly to do in the field or on the range, and risked losing bits like the nosecap muzzle bedding pad and/or spring, and/or securing screw in the long grass! Later rifle models had windows cut in the foresight protector so you could do it without that fuss, but it was still something done by the unit armourer, not Tommy.
Edited By Mick B1 on 13/12/2021 22:36:32