Well old chap, they're Spitfire MkIIA's, and judging by the colour schemes and the weather, I'd think it would be circa summer, 1941. The problem is, 74 Sqn used ZP as the Squadron codes during that period and, far as I'm aware, never used any codes with a 'L' configuration. So, the aircraft are either from a different squadron, with the second code letter obliterated under the orders of the censor (possibly 'LO', 602 Sqn, Aux AF, or 'KL', 54 Sqn.), or a temporary marking for whatever reason. Might even be a OTU or OCU, and the Life magazine people have used the pics, thinking, or having been told, that it's 74 Sqn?