[5e92cb50239222b]

Same. Not that it means much, but I've been seeing 51820 being used in all documentation, blog posts, etc. Probably just copypasting the original examples, e.g.

https://www.wireguard.com/quickstart/

https://www.wireguard.com/xplatform/

[chupasaurus]

A month ago I dug through the history, the only trace to origin of using 51820/UDP I've found is that the number is hardcoded in one of the tests.