Further to my previous post on this subject, I looked into a Seaside-2.8.4 image
 to find that:

1. The method, asComponent, is in the class Object.

   Object >> asComponent
    ^self description asComponentOn: self

2. The method, asComponentOn:, is in the class MAContainer.

   MAContainer >> asComponentOn:
    ^self componentClass
       memento: (anObject mementoClass
                 model: anObject
                 description: self)

I haven't found these methods in Pharo3/Seaside3/Magritte3.

How are components created from descriptions without them?