John,
to be completely honest with you I do not think that your badge is original, I have a photo on my office wall of a gathering of Guards Brigade Regimental Sergeant Majors taken around 1918, amongst them is the Sarnt Major of the MG Bn wearing what looks like the whitemetal one piece badge (as in my album) the photo is B&W so I cannot be 100%. As to the theory that it could have been made for wear by a soldier to be different, that just would never have happened in a Guards Regiment! If I had to guess I would say that it was a possible keepsake or sweetheart type badge, but not for wear in service. My opinion only!
All the best - Andy
