Went back to 
the park today and found more of these strange seed pods (I now know they aren’t seed pods). I examined all the trees around, but nothing came up that I could link to these things. Only Oaks, Beeches, Tilia, Cypresses, Cherry Laurel, …
They’re galls from a wasp named Andricus quercuscalicis which is known to create Knopper Galls.
I posted about my attempts to identify these with AI before, but Gemini, ChatGPT, Claude and even ObsIdentify failed. After cutting one open, looking closely, sitting back to think, I researched galls on quercus and these came up. Happy to know.