This was promulgated in Army Council Instruction 381 published on 3 April 1917. As in the other arms, ACI 380 also dealt with changes in the arrangements for TF records, pay and other administrative matters. These arrangements were to take effect from 1 Apr 1917.

It also set out the affiliation of Yeomanry units serving overseas to particular Reserve Cavalry Regiments, which were to act as reinforcing and rejoining units for them and it stated that soldiers serving in the Mounted Brigade Signal Troops were to be renumbered in the block assigned to the Yeomanry Regiment from which the troop was originally drawn. Under the Corps Warrant, Yeomanry units belonged to either the Cavalry Corps of Dragoons, Lancers or Hussars (as did regular cavalry units).

By the time this ACI was published a number of Yeomanry units had converted to infantry. ACI 381 emphasised that Yeomanry renumbering did not 'apply to personnel of the Yeomanry who have ceased to be Yeomen in consequence of their unit to which they belonged having been organised on an infantry basis, 3rd line Yeomen who have been transferred to Reserve Infantry Battalions, nor those transferred to Reserve Cavalry Regiments'.

TF soldiers retained their new number as long as they served in a unit of the same cavalry corps, even if posted to a regular unit of that corps. If they were transferred to another Cavalry Corps, they were transferred to a Yeomanry unit of that corps, renumbered in that unit's allocation, then could (if necessary) be posted to a regular unit of that corps.

Those soldiers treated as regulars for renumbering retained their existing number as long as they served in the same Cavalry Corps, even if subsequently posted to a TF unit of that Corps or to another regular unit of that corps. If they were transferred to another Cavalry Corps they received a regular number in their new corps, even if posted to a TF unit.

From date of renumbering the only men who could join Yeomanry units and be dealt with as TF were

The overall allocation of numbers was -

	Regulars	TF
Lancers	Below 30001	Above 30000
Dragoons	Below 70001	Above 70000
Hussars	Below 200001	Above 200000

The detailed allocation of numbers to Yeomanry units, together with affiliations to Reserve Cavalry Regiments and other details, are shown in the table below.