Light Trench Mortars - Infantry

Langleybaston1418 is correct: A Light TM Battery was found in each infantry brigade. These were manned by infantrymen and bore the same number as the brigade, i.e. 88th T. M. Bty. served with 88th Brigade in the 29th Division.

Medium / Heavy Trench Mortars Royal Artillery

Three Medium TM Batteries (later two) served with each division under the divisional artillery manned by RFA gunners. These were designated by the letters X, Y, and Z followed by the number designating the division, i.e. X/29, Y/29, Z/29; in 1916 and 1917 each division had a Heavy TM Battery which also served under the divisional artillery manned by RFA and RGA gunners. These were designated by a V followed by the number of the division, i.e. V/29.

Regards. Dick

Source: T.M.B.