1923/4 aren't particularly unusual on 15s though, they've been used on there everyday more or less for the last past few months. It frees up 2 Pulsars to cover X1 and 5 etc stuff like that.
There was Pulsars on 28/29 today too, along with Prestiges on 17, 17A and 17Bs as yesterday.
It'll take a few days for allocations to settle down while we find out if there are any specific issues with loadings etc. For example the school run to Egglescliffe on 17 may well be allocated a Prestige next week until we find out how many kids will use it. It got full before on a Cadet on the X6 and now it serves Thornaby too...