Let's Design and Build a (mostly) Digital Theremin!

Dewster, these images help a lot! Many thanks.

Finding the TOSLINK_TX part is difficoult.
post 1878: Buggins Q: "What parts are used in your design as optical (toslink) transmitter and receiver?"
post 1879: Dewster A: "https://www.goldmine-elec-products.com/prodinfo.asp?number=G16911B" with reference to Roger (pitts8rh) for the footprint.
This part is no longer available.

Just be the look of it, comparing with fotos of the main PCB, this ebay link may be it. These parts are obviously second choise with bent pins. Anyway I'd have bought them, but the parts will not be sent to Germany.
https://www.ebay.de/itm/111692790134?srsltid=AfmBOorU-U-4gMkfkj_Aws3tB-Ft9HFrAEjeiS5_0Bl4NWN_0WJSopwv

I may have to go for a different part Toslink transmitter and update the footprint.
As an alternative move the Toslink transmitter to the IO extension and connect it with a ribbon cable.
This could be the alternative: https://docs.rs-online.com/d4d9/A700000009791094.pdf

"Finding the TOSLINK_TX part is difficoult."  - gerbold

Yes, especially finding them for not outrageous prices.  Here are a couple on the US portal of eBay, doesn't say they do/don't ship to Germany:

https://www.ebay.com/itm/125935980277
https://www.ebay.com/itm/283080774405

Amazon:
https://www.amazon.com/dp/B0B1YGV918
https://www.amazon.com/dp/B09SNW3FLJ

You could just leave the TOSLINK TX off and use the electrical SPDIF connection on the expansion port.  SPDIF works just as well, though it doesn't provide the electrical isolation of TOSLINK (only a possible issue if you experience ground loop hum).