they used a thorn.

its an old english character that is 'th'.

https://en.m.wikipedia.org/wiki/Thorn_(letter)

Sort of similar to how if you read say, the Declaration of Independence, you will notice many instances where... what we would nowadays render as 's', gets rendered as a long s ( ſ ), like an f without the crossbar, sometimes in cursive, almost like the integral symbol ( ∫ ).

https://prologue.blogs.archives.gov/2021/12/14/the-long-s/