Complex sentence: Because of the encapsulated nature of Groucho's scenes, it had been assumed that his presence was an afterthought; however, recent Groucho letters show that he was to have been part of the project from its earliest stages in 1946-47.
Split into simpler sentences: Because of the encapsulated nature of Groucho's scenes, it had been assumed that his presence was an afterthought. However, recently discovered letters from Groucho show that he was to have been part of the project from its earliest stages in 1946-47.