What is the significance of a trailing u or d after the project's object ID in the URL?
The trailing letter indicates the scoping.
The object ID in the URLs references the currently scoped project.
For illustration purposes, to check the ObjectID in the URL against the ObjectID of the current project we start by installing a custom app that displays ObjectIDs of current workspace, current project, and current user. You may follow along in this article without installing this app in your workspace, but if you choose to install it, see KnowledgeBase article "How to Find the OID (Object ID) of Projects and Workspaces"