And interestingly enough, is also rejected by ISBN searchers!
…just like the seven ISBN numbers I’ve just found on the boxes of Legend Entertaimente IF releases. This is Eric the Unready:
Of course, all of them not recognised as valid numbers!
I guess now we can speculate that putting ISBN numbers in video-games was not such an uncommon practice back then… unlike actually registering them which… no one ever did? 
To add some more random chaos, there is at least one set of alternative ISBN numbers in some Infocom grey-boxes ¡yeah!
Commodore was known to support the release of new computer models licensing some existing games to publish on their own. That includes some VIC-20 cartridges of Scott Adams adventures or the well known Infocom blue-carpet releases for the C64.
It also happened with the less known Commodore Plus 4 model. Commodore just took a few greyboxes and released them with new stickers at the spine with their logo and a new variant of ISBN numbers, this time with 88731 as the publisher ID. This is Starcross:
I don’t know exactly how many of these C+4 variants existed. I’ve got a total of 4 and the biggest code is T266006 (Suspended). As usual, the ISBN numbers are not recognised.
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