As there are no indents for a tool, I doubt that it's screw off.
If you run a nail round the edge is it possible that a thin blade could get in all round the back rather than having a specific indent to get in on one place?
I've replaced the battery on my Oxford bike clock a couple of times but that's a simple 4 philips screws to get the back off then a miniscule screw that looks like a dust particle that keeps the battery in place