This plaque is at No. 29 Westmoreland Street, across the road from plaque 4. Harrison’s is long gone but there is still an eating house there viz. Charlies Chinese.
Perhaps you can get turtle soup there but certainly not mockturtle. Mockturtle soup in 1904 was a cheap imitation of Green Turtle soup. The main ingredients were the cheap leftover cuts of beef, particularly head and feet!!
Have a look at Heston Blumenthal’s adaption http://www.youtube.com/watch?v=PrGN5LmCQag