You may well be right, but the only full length 2.850" .458 I have seen here in the UK was for the Watts chamber, a drawing I have seen in a 1973 edition of Guns and Ammo in an article by the Jack Lott called for a 2.800" chamber.

The A2 boxes of ammo I had were all headstamped 458 Lott, in one of the 20 round boxes I opened it contained 4 cartridges with the over long cases. A second box from the same batch contained one such example which I still have. It has to be said this was some years ago, maybe things are better now.