I agree that the buses at EC are almost certainly for the 137 and will disappear in due course as the cascade of NB4Ls proceeds.
There are only 17 or so NB4Ls left that are unallocated. There is no evidence at all that they are going to the 468 which has a far higher PVR of 29. The contract award notice says that new hybrid double deckers will be bought for the 468. If NB4Ls were to be used the contract award would specifiy this. On another forum it was stated by an Arriva employee that HVs would be used on the 468. This would ensure a common type with routes 2 and 249 at Norwood garage. Arriva seem to be restricting the vehicle types used at garages to keep maintenance and spares costs under control.
The information I quoted was from the LOTS website, dated yesterday. It surprised me, because I'd have thought the 68, moving from London Central at the same time as the 468, would have been a more 'natural' home, given it is truly central London and parallels the 168 and 91 from Euston to Aldwych, but maybe Abellio couldn't cope with NB4Ls at Walworth. Norwood, on the other hand, has only fairly recently gained experience of the type, with some transferred from Brixton, so there is some logic there. The 468 used to be run by Arriva from Croydon, of course, but that was when they had the 68 too, from Norwood. By the by, a chance to extend the route to TC garage again.
On the matter of how many LTs are unallocated, the cynic in me says there may well be further pruning to achieve the desired number!