They are known, but there are multiple different forms. Some of the forms may have never been seen, and some of them cannot be expressed in the Unicode, as it was made with Latin letters in mind.

So when you're trying to digitize abiguda, you have to be careful about ligatures, because real world may have multiple different forms in different context, and you can get to choose only one. But when we are talking about archiving, it has to be perfectly copy-pasted the way it was in the palm inscription.