https://www.doorbird.com/downloads/datasheet_dkh_surface_en.pdf
Doorbird Intercom D21xKH Surface-mounting housing (Backbox)
$289.51
Stainless steel V4A (salt-water and grinding dust resistant)
Stainless steel V4A (salt-water and grinding dust resistant), brushed (GTIN/EAN: 4260423867529, P/n: 423867529)
Fits only a D21xKH IP Video Door Station