This is the base class for CUISerializer elements that can be versioned.

Autodesk.AutoCAD.Customization.ElementBase
    Autodesk.AutoCAD.Customization.VersionableElement
        Autodesk.AutoCAD.Customization.CustomizationElement
            Autodesk.AutoCAD.Customization.ButtonGroup
            Autodesk.AutoCAD.Customization.CustomizationReference
            Autodesk.AutoCAD.Customization.DoubleClickAction
            Autodesk.AutoCAD.Customization.ImageMenu
            Autodesk.AutoCAD.Customization.MenuMacro
            Autodesk.AutoCAD.Customization.ObjectType
            Autodesk.AutoCAD.Customization.Panel
            Autodesk.AutoCAD.Customization.PanelSet
            Autodesk.AutoCAD.Customization.PopMenu
            Autodesk.AutoCAD.Customization.PopMenuItemBase
            Autodesk.AutoCAD.Customization.QuickAccessToolbar
            Autodesk.AutoCAD.Customization.RibbonItem
            Autodesk.AutoCAD.Customization.RibbonRoot
            Autodesk.AutoCAD.Customization.ScreenMenu
            Autodesk.AutoCAD.Customization.TabletMenu
            Autodesk.AutoCAD.Customization.TemporaryOverride
            Autodesk.AutoCAD.Customization.Toolbar
            Autodesk.AutoCAD.Customization.ToolbarItemBase
            Autodesk.AutoCAD.Customization.Workspace
            Autodesk.AutoCAD.Customization.WorkspaceDockableWindow
            Autodesk.AutoCAD.Customization.WorkspacePopMenu
            Autodesk.AutoCAD.Customization.WorkspaceQuickAccessToolbar
            Autodesk.AutoCAD.Customization.WorkspaceToolbar
            Autodesk.AutoCAD.Customization.WSRibbonPanelSourceReference
            Autodesk.AutoCAD.Customization.WSRibbonRoot
            Autodesk.AutoCAD.Customization.WSRibbonTabSourceReference

Public Class CustomizationElement
Inherits VersionableElement

public class CustomizationElement : VersionableElement;