Many years ago, I ran into the question as to why and when men of the Territorial Force units of the Army Service Corps were given new numbers prefixed with T4. I enquired at the Great War Forum and received this kind reply from member Graham Stewart:
30th August 1916
Army Council Instruction 1677. Army Service Corps T.F. Posting to Regular Army Service Corps:-
(1) Owing to the complicated situation which has already arisen abroad and which may be further complicated as the result of the necessary local allotment and re-allotment of the T.F. Divisional Trains to reconstitute Regular & TF Divisions with varying War Establishments for the trains, it is frequently impossible to say whether demands for A.S.C. reinforcement from an Expeditionary Force are required from Regular or T.F. personnel.
(2) The Army Council having carefully considered the question, are of the opinion that it is essential from the point of view of the efficiency and economy in personnel to have a general “pool” of Regular A.S.C. personnel in this country in order to maintain and reinforce the A.S.C. units – Regular & T.F.- serving abroad.
(3) In order to give effect to this system and to make the best use of the A.S.C. personnel, both Regular & T.F., it has been decided to post all N.C.O.’s and men of the T.F. to the Regular A.S.C. for the duration of the war, whether they be serving at home or abroad.
(4) The posting will take effect from 1st September 1916, the procedure laid down in A.C.I.1166 of 1916 being followed.
(5) A nominal roll of all officers, showing the duties it is proposed to assign to each in accordance with War Establishments, should be forwarded, as early as possible, to the War Office by the G.O.C.-in-C. under whom the unit is serving, in order the necessary Gazette notices be carried out.
(6) On the termination of the war, officers will have the option of rejoining their original units should they so wish. N.C.O.’s and men whose period of engagement in the T.F. has not expired, will be re-posted to their original unit.
9/A.S.C./2642(A.G.2B(S)).
This was followed by a large Memorandum, one interesting note being:-
“After posting to the Regulars, should it be desirable to retain some distinguishing mark to show their origin, there would be no objection to the continual wearing of the letter “T” by officers and men so dealt with.”