I agree with Extremist. I think he is correct on the original finish being blue. The two guns shown are only about 100 s/ns apart, in the 171xxx-range.
Curiously both of them are much later guns, and I was not aware that Walther had received another contract from the British for additional deliveries. The initial contract, which was let only after protracted and very extensive testing, was for only 2,300 units, followed shortly thereafter by 200 more. Commercial deliveries to Interarms followed. Surprisingly the very first batch of commercial guns were made up on overrun British contract slides, and bore the NATO stock number (but commercial Ulm proofs). These guns, and the ones that immediately followed (with polished slides and no stock number) were in the 151xxx serial range. Initially these came with laminated wooden grip panels, which broke so distressingly quickly that Walther abruptly discontinued them. The early guns also had a trigger stop milled as a stud on the back of the trigger, as in the standard P5, and the backstrap was of light alloy, with vertical striations. By the time s/ns reached 152xxx, the trigger stop had been deleted and the backstrap was plastic, with horizontal grooving, as clearly shown in one of the pix above.
Walther was very proud of this gun, as it had passed the most stringent requirements set down by the SAS -- apparently eclipsing all of its competitors. Evidently the Brits have liked it well enough to go back for more.
I don't know if you were responding to ME but I have no idea if it's unaltered. It could be. I was simply reminding everyone NOT to jump to "internet forum conclusions" since it could very easily be a shitty picture rather than a refinished weapon.