Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
Magic, thank you for the info, now off.Yes, it looks like a Mira one. Using a flat bladed screwdriver, or similar, prise in between the tiny gap and push it down (indicated on edited photo), this should hen reveal a screw.
Reply to the thread, titled "How to remove Shower riser rail" which is posted in Showers and Wetrooms Advice on Plumbers Forums.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.