diff --git a/src/calibre/gui2/complete2.py b/src/calibre/gui2/complete2.py
index f0145520a4..e7e6046e13 100644
--- a/src/calibre/gui2/complete2.py
+++ b/src/calibre/gui2/complete2.py
@@ -290,6 +290,7 @@ class LineEdit(QLineEdit, LineEditECM):
     A call to self.set_separator(None) will allow this widget to be used
     to complete non multiple fields as well.
     '''
+    item_selected = pyqtSignal(object)
 
     def __init__(self, parent=None, completer_widget=None, sort_func=sort_key):
         QLineEdit.__init__(self, parent)
@@ -407,12 +408,16 @@ class LineEdit(QLineEdit, LineEditECM):
         before_text, after_text = self.get_completed_text(unicode(text))
         self.setText(before_text + after_text)
         self.setCursorPosition(len(before_text))
+        self.item_selected.emit(text)
 
 class EditWithComplete(EnComboBox):
 
+    item_selected = pyqtSignal(object)
+
     def __init__(self, *args, **kwargs):
         EnComboBox.__init__(self, *args)
         self.setLineEdit(LineEdit(self, completer_widget=self, sort_func=kwargs.get('sort_func', sort_key)))
+        self.lineEdit().item_selected.connect(self.item_selected)
         self.setCompleter(None)
         self.eat_focus_out = True
         self.installEventFilter(self)
diff --git a/src/calibre/gui2/metadata/diff.py b/src/calibre/gui2/metadata/diff.py
index 6c19bbf00a..0f03c9ac54 100644
--- a/src/calibre/gui2/metadata/diff.py
+++ b/src/calibre/gui2/metadata/diff.py
@@ -6,7 +6,7 @@ from __future__ import (unicode_literals, division, absolute_import,
 __license__ = 'GPL v3'
 __copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
 
-import os
+import os, weakref
 from collections import OrderedDict, namedtuple
 from functools import partial
 from future_builtins import zip
@@ -18,7 +18,7 @@ from PyQt5.Qt import (
     QKeySequence, QAction, QMenu)
 
 from calibre import fit_image
-from calibre.ebooks.metadata import title_sort, authors_to_sort_string
+from calibre.ebooks.metadata import title_sort, authors_to_sort_string, fmt_sidx
 from calibre.gui2 import pixmap_to_data, gprefs
 from calibre.gui2.complete2 import LineEdit as EditWithComplete
 from calibre.gui2.comments_editor import Editor
@@ -193,6 +193,11 @@ class DateEdit(PubdateEdit):
 
 class SeriesEdit(LineEdit):
 
+    def __init__(self, *args, **kwargs):
+        LineEdit.__init__(self, *args, **kwargs)
+        self.dbref = None
+        self.item_selected.connect(self.insert_series_index)
+
     def from_mi(self, mi):
         series = mi.get(self.field, default='')
         series_index = mi.get(self.field + '_index', default=1.0)
@@ -208,10 +213,21 @@ class SeriesEdit(LineEdit):
             series_index = float(val.rpartition('[')[-1].rstrip(']').strip())
         except:
             series_index = 1.0
-        series = val.rpartition('[')[0].strip() or None
+        series = val.rpartition('[')[0].strip() or val.rpartition('[')[-1].strip() or None
         mi.set(self.field, series)
         mi.set(self.field + '_index', series_index)
 
+    def set_db(self, db):
+        self.dbref = weakref.ref(db)
+
+    def insert_series_index(self, series):
+        db = self.dbref()
+        if db is None or not series:
+            return
+        num = db.get_next_series_num_for(series)
+        sidx = fmt_sidx(num)
+        self.setText(self.text() + ' [%s]' % sidx)
+
 class IdentifiersEdit(LineEdit):
 
     def from_mi(self, mi):
@@ -410,6 +426,8 @@ class CompareSingle(QWidget):
                     neww.update_items_cache(db.new_api.all_field_names(field))
                 except ValueError:
                     pass  # A one-one field like title
+            if isinstance(neww, SeriesEdit):
+                neww.set_db(db.new_api)
             oldw = cls(field, False, self, m, extra)
             newl = QLabel('&%s:' % m['name'])
             newl.setBuddy(neww)