Pentagonal shaped cap badge ?

[GriffMJ]

Just done some further reading:-

A Royal warrant was issued on the 10th May 1918, constituting the regiment under the designation of the sixth Machine Gun Regiment of Foot Guards or Guards Machine Gun Regiment, which constituted of five battalions:

1st LG Btn (CO Lt.Col. E H Brassey) 40 Officers 720 ORs
2nd LG Btn (CO Lt.Col. A F Stanley) 31 Officers 822 ORs
3rd RHG Btn (CO Lt.Col. Lord Tweedmouth) 30 Officers 756 ORs
4th Foot Guards (CO Lt.Col R C Bingham) 59 Officers 983 ORs
5th Res Btn (CO Lt.Col. R E K Latham) 70 Officers 1,051 ORs