The SR-F25 is a sweet little horizontal shirt-pocket radio and another great design from Standard. While I have no specific data on a general release date for the model, the unit pictured here looks to have been built around July 1958. In addition, the model does appear in a magazine dated July 1958. This magazine would have been on the newsstands by early June '58, so data on the model would have been available to the editorial staff by May '58 at the latest.
This sample is missing the lettering from the top right corner, so I approximated it on this scanned image to give an idea of how it should look.