If you have a Scott Specialized Catalogue, look under the "Encased Postage Stamps" section for the history. Yours is Scott #EP158. If you don't have the catalogue, you can Google "Encased Postage Stamps" and get information about them. Value depends upon condition. You did not post a scan of the front. The Mica being intact is important to value. Catalogue value for a very-fine example, intact Mica and not heavily worn or damaged, is about $800.00 (my 2014 catalogue).
