Nice looking rifle!
I have read that Eley were the ones to originally develop and load the .404, it would then make sense that Lancaster marked the rifle as such.
The Kynoch, Eley, Kings Norton companies did produce bespoke cartridges for the major Brit arms makers.
.404 would have been understood to be a .404, just “branded” and (possibly) loaded to Eley specs which could have been slightly different from Kynoch or others.
Still, the “typical” .404 400 grain ammo was loaded either to tropical spec of 55 grains Cordite or the full 60 grain Cordite load.
There were other bullet weights offered as well and bullet weight not necessarily marked on the rifle, just the proofed loading.

The Ross….
According to the Fleming book.
There were several .280 cartridges loaded by Eley, Kynoch, and Kings Norton:
.280 Kings Norton Experimental .509 rim diameter
28 Ross (280 Roosevelt) ~.485 rim diameter
280 Westley Richards .468 rim diameter
280 Flanged Magnum (Rimmed Ross) .602 rim diameter
280 Ross .551-.558 rim diameter
-and-
280 Jeffery .532-.541 rim diameter

The 280 Ross was also known as the 280 Rimless Nitro Express
The original rimless 280 Ross had a rim slightly larger than base diameter .534 base/.556 rim

This is the “fun” of getting vintage Brit rifles shooting …….

- Mike