The gymnasium at the Royal Naval Barracks, Devonport (west Plymouth in Devon, England) shown on a postcard dating from about 1906 when the gymnasium was completed, with a space for correspondence marked "This space may be used for communication for Inland Postage only, NOT Foreign".
The men are standing in a most peculiar position. I don't know what sort of exercise it was, possibly for flexibility, but I feel sure the gymnasium will look very different these days.
A post for Sepia Saturday. A click on the button will take you to the blog.