I think this is him who I've found on the Absent Voter’s list Stalybridge and Hyde 1918 on FMP
William Russell, 294 King Street, Dukinfield, Cheshire 26538. Cpl.Mec., 49th Sqd RAF
He is also on the RAF muster roll on FMP, which gives the following info
Air Force Number 26538, Name William Russell, RNAS or RFC Classification - Rigger, Remustered to Air Force Trade Classification - Rigger (Aero), RNAS rating or RFC rank or Appointment = Corporal, New rank in Air Force - Cpl.Mech, date of joining 14/4/16, date of last promotion 1/10/17, Normal rate Air Force Pay 5s 0d, Terms of Enlistment D/W.
If it is correct and you would like a copy of the document please let me know and I will scan a copy to you, unfortunately I'm getting a message that the file is too big so can't post here.