Wendi.
It is all very confusing, differing between regular army, volunteer, territorial and militia battalions.
6th Manchesters in Stretford Road were originally the 6th Lancashire Rifle Volunteers in 1859, then in 1888 were retitled 4th Volunteer Battalion Manchester Regiment and then in 1908 retitled 6th Territial Battalion Manchester Regiment.
The 4th Militia Battalion Manchester Regiment was renumbered 6th Militia Battalion in 1900.
Brad must have a closer look at the attestation document to see if there are any better clues. Also whereabouts did his Grandfather live? That certainly could have a bearing.