It would likely have been done using a mold similar to the ones in this article about Roman breadmaking in Pompeii. [0] This story did make it to HN a couple or three years ago, maybe longer, and has been updated since.

[0]https://tavolamediterranea.com/2018/06/14/baking-bread-roman...

There's some bullshit about no part of the website being reproduced, etc at the bottom of the article but I ignored it since they posted it online where anyone can read it and they mentioned earlier traffic from HN so I'm sure they're totally okay with us abusing their materials, maybe as long as we all purchase something from their shops. /s