diff --git a/protobuf_definitions/message_formats.proto b/protobuf_definitions/message_formats.proto
index cc9b257f..7d7f5218 100644
--- a/protobuf_definitions/message_formats.proto
+++ b/protobuf_definitions/message_formats.proto
@@ -1122,6 +1122,9 @@ enum GuestPortDeviceID {
   GUEST_PORT_DEVICE_ID_BLUEYE_MULTIBEAM_SERVO_V2 = 46; // Blueye Multibeam Skid Servo V2.
   GUEST_PORT_DEVICE_ID_CERULEAN_OMNISCAN_450_COMPACT = 47; // Cerulean Omniscan 450 Compact.
   GUEST_PORT_DEVICE_ID_BLUEYE_SCALING_LASER = 48; // Blueye Scaling Laser.
+  // Cygnus Mini ROV Thickness Gauge on a connector that converts its RS485 to
+  // RS232, letting it be used on drones with no RS485, such as the X7.
+  GUEST_PORT_DEVICE_ID_CYGNUS_MINI_ROV_THICKNESS_GAUGE_RS232 = 49;
 }
 
 // Guest port number.