Final answer:
Josephine had taken on the identity of the Masked Rider, a famous figure associated with the rustlers. She had dressed as a boy and disguised herself with a mask to carry out her activities as the Masked Rider.
Step-by-step explanation:
When Josephine was asked by Aven if she remembered another girl at Stagecoach Pass in 1973 who looked like her but had arms, it was revealed that Josephine had taken on the identity of the Masked Rider, a famous figure associated with the rustlers. Josephine had dressed as a boy and disguised herself with a mask to carry out her activities as the Masked Rider. This was done in order to help protect her true identity and maintain secrecy in her actions. The question implies that Josephine had been living a double life, pretending to be the innocent and gentle Josephine while also acting as the daring and assertive Masked Rider.