Exterior Service Window fits into rough wall opening and clamps onto wall. Adjustable extruded aluminum frame fits walls 5" to 8.5" thick. Window clamps in place with bolts positioned through back frame into threaded section of front frame. Glazing is sealed into the frame for weather tight installation. Stainless steel shelf has writing surface in front of built-in deal tray. Deal tray closure helps prevent drafts. Natural voice transmission is provided by the circular draft-free talk thru.