After 90 minutes of scouring the net, I found nothing. Most of this you should already have:

Apple's archived technical sheet on the 550c: http://support.apple.com/kb/sp164

Apple's take apart manual for the 550c: http://tim.id.au/laptops/apple/powerbook/powerbook_550c.pdf

Some user's PB500 Shrine, with a few 550c links from Japan: http://web.wt.net/~kikuko/PB500.html

---> There is also an article there about PowerPC upgrades in the 500 series burning out, its at the bottom of the page.

==> Some links come up as 404, so its hit and miss there.

Apple Developer's Notes Archive on "powerbook 55c" search:

No Documents Found

The entire search term must match for an entry to show.
If you entered more than one word, try again with fewer words.

I can guess that Apple-Japan might have it but I'm in the USA and do not read Japanese.

Sorry with coming up empty handed.

~Elfen