https://www.doorbird.com/downloads/datasheet_d21dkv_surface_en.pdf
Doorbird Intercom D21DKV Surface-mounting housing (Backbox)
$367.95
Stainless steel V4A (salt-water and grinding dust resistant)
Stainless steel V4A (salt-water and grinding dust resistant), brushed (GTIN/EAN: 4260423868533, P/n: 423868533)
Fits only a D21DKV IP Video Door Station