There seem to be quite a few different releases with different content.

There are the (at least to me) most common versions from Software Corner
Dragon's Lair [SC 426020-1]
Thayer's Quest [SC 426020-2]

Mine came together in a box:


With an additional picture:


The box has room for much more, so maybe some extras are missing.


Then there are two RDI discs but it is unclear where are the differences.
590-0002 has a metal plate on the dead side. Likely for an arcade machine. Never seen a 590-0001.


Then there is a two sided release that starts with Halcyon and RDI logos.


The two labels are slightly different. I think, the content is the same but the sides are reversed between the two discs. I've only done a short check.

So far I haven't found any other Dragon's Lair discs.

Does anyone know more about the different Thayer's Quest versions?

I have no clue but there's a least one more: Thayer's Quest Arcade Version [08563]



Julien

The first SC Dragon's Lair discs (the imported UK ones) seem to have a different label.

Source: https://www.ebay.de/itm/356599727101

Regular label for comparison.