See 2.2.52.1.8 of ADN:

"Classification of organic peroxides not listed in 2.2.52.4, 4.1.4.2 of ADR, packing instruction IBC520 or 4.2.5.2 of ADR, portable tank instruction T23, and assignment to a collective entry shall be made by the competent authority of the country of origin. The statement of approval shall contain the classification and the relevant conditions of carriage. [...]"

This BAM No. represents the collective entry itself - it has been assigned packaging method OP7 according to 4.1.7.1.4 c) of the ADR.