[Zope] Custom ZClasses absent from addmenu after upgrading

[pmiller@liverpool.ac.uk]

I am in the process of upgrading a site on NT from 2.3.3 on one
machine to 2.5.1 on another. I copied across the data.fs, reinstalled
a few products and everything seems to be working OK apart from the
addmenu which stubbornly refuses to list a number of custom ZClasses.
They list OK in the Control Panel, existing instances still work and I
can copy/paste to create new ones. Any suggestions much appreciated.

Peter