“Sons of Perdition” SEC Filing? New Play/Musical?

Based on the filing, it looks like this is the Matt Stone, of South Park and Book of Mormon fame. Anne Garefino is Stone and Trey Parker's producing partner, she's worked on South Park and Book of Mormon.

Could they be doing a follow up to Book of Mormon?

It’s very possible that this entity is merely a raise for a new mounting of MORMON (on tour or elsewhere).

The way an entity is named is at the discretion of the producer and only references the title about 50% of the times. Some producers like something straightforward (I.E. Night Music Broadway LLC) while some others like to get cute (Brigand Mustache LLC).

Call_me_jorge said: "I wonder if it could be for a potential pro shot? There’s a non-equity tour running with dates through next summer, so another equity tour wouldn’t make sense.

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That would make sense. If it's a new musical or even a new tour, $5.9 million is pretty cheap, especially considering the original production cost $11.4 million ($16.3 million adjusted for inflation).