No, this is a secret treasure, so even we are seeing it for the first time on this occasion.
Literal
No, this [topic-は] secret-treasure [copula-ですから]. We [also-も] this occasion, first-time saw [copula-のです].
秘宝 'secret treasure' (秘 'secret' + 宝 'treasure') is a formal/literary compound. わたくしたち is the most polite first-person plural (more formal than わたしたち). この度 (このたび) 'this occasion / this time' is a formal temporal expression. The whole tone is hushed and reverential — museum or temple context.