What API is responsible for 'making single user' for node groups and materials?

Archive Python API
1

The popovers of corresponding buttons do not contain any python reference.

Function id_single_user from source/blender/blendkern/intern/library.c is called from some weird places:

singleuser_action_cb
    outliner_id_operation_exec
        OUTLINER_OT_id_operation
singleuser_world_cb
    outliner_id_operation_exec
        OUTLINER_OT_id_operation
template_id_cb
    template_ID(7 usages)
        ui_template_id
            uiTemplateGpencilColorPreview
            uiTemplateID
            uiTemplateIDBrowse
            uiTemplateIDPreview
            uiTemplateIDTabs
    template_id_def_new_but(2 usages)
        template_ID
            ui_template_id
        template_ID_tabs
            ui_template_id
2

A quick grep thru the code suggests that id_single_user(C, id, &ptr, prop) may just be a C function without a bpy wrapper.