Anson was originally an "Intrepid" class
The drawings for the Intrepid don't show that port either, hehe. So I would suppose it may have been either painter's fantasy (the second picture of the same author attributed to almost the same date do not show it!) or a captains' decision at certain point. I just like how it looks.
The existing "HMS_Indefatigable" and "FR_Indefatigable" are being renamed "RN_Razee" and "FR_Razee".
Since it's a second model of Unite class, I would suggest some other name for the class. I would give Indy's new model to represent the razees for the moment in other navies (while in reality they all were very different, according to the base class, we don't have the models and in most cases even drawings), to represent something in between ships of the line and frigates, and move the existing model to a place of a light frigate she really is:
HMS Surprise (1796) - Wikipedia