In the 1914 Drill Manual (itelf based on earlier editions), there is the following:-

"...The firing party, consisting of 1 serjeant, 1 corporal and 12 privates, will be drawn up two deep, one pace interval between files, facing the building where the body is placed. The corporal will be on the right of the front rank. The serjeant will give all words of command, and be posted in rear of the centre. Arms will be at the slope..."

So there is a firing party of 13 men -

I assume this is the
..."Three rounds blank" an' follow me,
An' it's "Thirteen rank" an' follow me...

Any help?

Tom