The heyday of the tintype photograph was in the 1860s and 1870s, however the technique continued to be used as a form of instant photography at fairs and on piers into the middle of the twentieth century. Tintype photographs used a thin piece of iron with a photo-sensitive lacquer that captured the image when the photograph was taken. The resulting photograph could be set and given to the customer within a few minutes without the need for chemicals, paper and darkrooms. As such it was still being used in the 1940s in certain parts of the world for tourist photographs and today, we are looking at a very small example of these tin type photographs depicting a group of RAF servicemen: 

The men are wearing great coats, with FS caps and all have respirators slung across their bodies: 

The most likely explanation for this photograph then is that the four airmen were offered a photograph from a commercial photographer whilst on a leave pass and this little photograph is the resulting souvenir. It is certainly very unusual and I haven’t found many references to tintype photographs as late as this, but clearly they did exist as this image proves.