@timtfj Well, Johannes Gutenberg was German. Did German use thorn?

It doesn't look like Old High German used thorn:
https://en.wikipedia.org/wiki/Old_High_German#Consonants

I think languages in the Netherlands and Belgium were closer to German than they were to English. And England back then had very close trading ties with the Netherlands

So maybe the letter set the printers brought over to England didn't include thorn to start with?