Panatic,
The "Y501Y" is actually the version.
I believe that's one of the early ones,
maybe a -1. It was manufactured March 25 of 1999.
I would open it up and see if the Eprom chips are marked.
Also, you may have a later version of a KX-TA 62477 3X8 expansion card, which will support the backlit sets.
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MrG