The enigmatic 1856/5 overdate is one of the more difficult overdates of the Sydney mint series. The overdate isn't as clearly visible as other issues such as the 1861/0 overdate, features hints of the loop of the 5 on the lower-right hand corner of the 6 but otherwise is chiefly identified by the presence of a dot after the date, a variety otherwise unique to the 1855.
The dot is not as clear on the overdate as on the 1855 no doubt due to the additional re-working needed to change the date and may be worn away on lower grade examples making attribution difficult in such cases. It is also likely that given the amount of die re-working undertaken by the Sydney mint, that the dot has completely worn away on some later strikes. As the dot serves as the main diagnostic criteria, such examples should be viewed with caution.