This 20p coin does not have a year shown.  In 2008, the reverse of our coinage changed from the Britannia to the Shield.  Also, the Queen's portrait changed from her 4th portrait to her 5th portrait.  This coin should have been struck with the 5th portrait (thus showing the year 2008), but the wrong die was used (ie 4th portrait).  Hence the year is missing.

Please see photos for condition.