Every oil mist detectors has two separate serial numbers, one for the measuring head and one for the connection box/vault box. The measuring head serial number can be found in the display for VN/87plus and VN/93 models, and in the measuring head label for VN/87 EMC models. The connection box/vault box serial number can be found on the unit label for all models.
See images below for exact locations.
VN 115/87 EMC, VN 116/87 EMC & VN 215/87 EMC
VN 115/87plus, VN 116/87plus & VN 215/87plus
VN 115/93, VN 116/93 & VN 215/93