*sticking my neck out*

1/8 em quad (as an approximation).

Type setting is a minefield with every language having their own lil quirks, e.g. in French type setting, that’s just called a “thin space”.

There’s a bit more info here: https://en.m.wikipedia.org/wiki/History_of_se...

Hier heißt es :

Typographical Printing-Surfaces

https://books.google.co.uk/books?id=yUkHAwAAQ...

 ... an eighth-em ... ein Achtelgeviert ...

Standard Dictionary of Advertising, Mass Media and Marketing / Standard Wörterbuch für Werbung, Massenmedien und Marketing: English-German / Englisch-Deutsch

Wolfgang J. Koschnick - Walter de Gruyter GmbH & Co KG, 24.05.2016 - 476 Seiten

https://books.google.co.uk/books?id=6Zt7DwAAQ...

 ... em-space Achtelgeviert n (Typographie) ...