Board Thread:General Discussion/@comment-110.146.149.161-20150813074512/@comment-4010415-20150907230232

They might've already been working on the script while Monty was making the rest of the trailers. After all, the Yellow Trailer and V1E1 were made at the same time. (Source: Volume 1 director's commentary) In fact, Lindsay even voiced her lines for V1E1 before she voiced her line for the Yellow Trailer, if I remember correctly.

So, it was likely at some point while the script was being made, before the White, Black, and Yellow Trailers and episodes were animated, but still late enough where the picture at the ends of the trailers depicts Weiss holding Myrtenaster in her right hand.