Help authenticating this IWC mk11

Hi everyone, 

This is an Mk11 that I am about to get but I have doubts on the originality of the whole and specifically the case back.

Dial, case and movement all match a military issued Mk11. But the case back is not. From the feedback I got from another (excellent) community, it could have been put together in the 60s by the RAF Lightnings, maybe for the Saudi Air Force Lightnings. But the movement serial makes it from 1949, which is much older and matching a RAF issued Mk11 in the late 40s or early 50s. 

Also, the IWC engraving inside the case back is cut and overlaps with the inside disk.

So what do you think? Shall I stay away from this watch? 

Thanks. 

Indeed the back doesn't belong to a mil. Mk. 11. Without having the back of the watch in flesh on the desk it is hard to decide, whether or not it is a complete fake back, a back from a camera watch with the markings skimmed or a civilian case back of the 1,726,xxx case number range.

Could you please provide the case number engraved under the disc? It starts 1726, but the last three digits of the case number are not legible on your pics. With the full case number at hand, I can have a look whether I have some info on this specific case number. 

Regards

Thomas Koenig

Thank you. 

These are better pictures of the case back. The serial is 1726831.