This black steel flange is Harvia HL1M for Virta heaters in the HL70–HL110 class. On this shop: HL60E, HL80E, HL90E, HL110E. Not Virta Pro 16 / 20 (the SKU does not name those). Not Combi.
How it is used
Bench cutout, Virta through the hole, black flange on the rim. Harvia lists embedding flange, install sheet, and screws in the carton.