CustomAcion (NewMenu) in a specific user-defined list

Question

I like to create a CustomAction an add this one to the NewMenu of a specific list.

<CustomAction
   ...
   RegistrationType="List"
   RegistrationId="100"
   GroupId="NewMenu"
   Location="Microsoft.SharePoint.StandardMenu"
   ...
   >
</CustomAction>

This code snippet adds the CustomAction to all custom-lists (100). What if I want a specific one?

I tried to use the ContentType (id detected via url)

<CustomAction
   ...
   RegistrationType="ContentType"
   RegistrationId="0x010100C568DB..."
   GroupId="NewMenu"
   Location="Microsoft.SharePoint.StandardMenu"
   ...
   >
</CustomAction>

The problem is, my List uses more then 1 contenttype. If i like to add it to Location="EditControlBlock" all went fine. But not with GroupId "NewMenu"!

Any solution?