positive written identification

[Femme]

the context is as follows:

In order for such information and data to be considered PROPRIETARY and subject to this AGREEMENT, it shall be identified in writing at the time of the disclosure by an appropriate legend, marking, stamp or other positive written identification to be PROPRIETARY.

Would appreciate your help:)

[Slava]

Имхо, можно написать просто "письменное подтверждение".