As found via the Known World Scribal List (which is awfully quiet anymore),
Aryanhwy has done an amazing job
indexing "Puzzle Initials" from the British Library.
Her generous project introduced me to the term "Puzzle Initial," though I'm far from an expert in this style. I did, however make some inquiries a number of years ago about this style via the same list when it was more active. I've heard the terms "Egg and dart," and "buttonfern." Obviously Puzzle Initial is the term used by the British Museum, making it far more likely to be official.
The speculation given to me years ago was that as the calligraphy was typically immaculate on this style, and there was little illumination, so it was possible that many illuminators died in the plague, giving birth to this style. A pretty bit of storytelling, but the dates don't quite line up with this sad tale, as the above example is from
Valerius Maximus's Facta et dicta memorabilia, c. 1471. The Black Death peaked in Europe between 1348 and 1350, killing between 75 million and 200 million people.
Whatever its true history, I find this style to be elegant and both complex and simple in one fell swoop. Aryanhwy's index is a great tool for scribes to create versals, and you can bet I'll be using it in the future. Thank you for this great tool and resource!