What's cool is the wide variety of dress in WWI, with the Scottish pattern jackets, English pattern, and modified English pattern all worn with the kilt, and gas mask bags worn as sporrans etc.
These matters were oftentimes regularised within certain units, for example I saw a photo of various members of a certain Highland battalion, all the members wearing their gas mask bags as sporrans in precisely the same manner, while members of a different Highland battalion in the same photo are all wearing no sporran or bag.
This is an extemely interesting photo which rewards a bit of study: