Can't you just ask for their revenue and expenses? I've never had to do so myself, but I believe you can simply ask for a copy of their 'form 990', which you're entitled to for any 501(с)(3) charity such as the Wikimedia Endowment.

I'm actually rather curious now. Let me know if you write, because I'd like to avoid the airmail fee from the UK if at all possible :)

There is no Form 990. The Endowment was set up as a Collective Action Fund with The Tides Foundation. The Wikimedia Foundation promised many times that it would convert the Endowment into a standalone 501(c)(3) filing an annual Form 990 but then delayed and delayed. (Even now, one year after IRS approval for the 501(c)(3), the Endowment website still says it is with Tides.)

https://wikimediaendowment.org/

I have asked for revenue and expenses information (many times in fact), so far without success. Feel free to join the ongoing discussion on Jimmy Wales' talk page on Wikipedia:

https://en.wikipedia.org/wiki/User_talk:Jimbo_Wales#Endowmen...

(The above link will decay once the discussion on Wales' talk page is archived. Here is a permalink: https://en.wikipedia.org/w/index.php?title=User_talk:Jimbo_W... )

Thank you for explaining this. Looking through the Tides Foundation's own financial reports, I can find only aggregated reports, except for Tide's own Form 990 which says that they gave $3M back to the Wikimedia Foundation in their 2021 financial year (this disclosure I imagine is for tax rebate purposes?).

In any case, thank you for saving £2.20 of my own expenses and the IRS a letter asking about a non-existent 501(c)(3) ;)

You're welcome. :)

Yes, in the Tides Form 990 nothing is broken down, so you can't see what the revenue and spending was. Jimmy Wales actually acknowledged that in this discussion:

https://en.wikipedia.org/w/index.php?title=User_talk:Jimbo_W...

But that is all he said ... we're still waiting for proper disclosure of over 7 years of financial records ...