Unit ID - photo

[4thGordons]

Just picked up this rather battered picture.

French postcard back and he has his (slung) SBR so I assume taken relatively close to the Front (see also relatively disheveled look)

Can anyone shed any light on which particular unit this might be? Perhaps based on the leather sporran? I can make nothing useful out of the collar or shoulder brasses, nor the badge on the Tam. He appears to have a marksman's badge on his arm and his tunic is not a standard one (see pocket flaps, apparent lack of rifle patches but with pocket pleats)

Thanks in advance

Chris