This one is a bit of a puzzle. A number of sources mention three of the graves represented here: Prudent-Villers (far left), Mortier (next to Prudent and Clement (far right) but not the name of the grave with the bust of a man.and to make matters worse, the bust in the postcard is missing.
However, there is a possible answer to this tiny mystery. In Moiroux's 1908 guide to the cemetery he mentions the grave of French painter and former communard Louis-Ernest Pichio (d. 1898) as being near this very location.
Pichio, also known as "Picq", was perhaps most famous for his painting of the summary execution of 146 communards at this very spot on May 28, 1871.
I have a number of vintage postcards of various views of Avenue Principale in my collection but wanted to share this one since it shows the Avenue before the center portion was dug out.
When the cemetery was first opened in 1804 this portion was quickly filled with graves, all of which are now long gone, either lost or removed to the ossuary. It is quite possible that the three pieces of stone or marble in the near center portion of the grass might be old headstones.
Also you get an idea of what the original view of the top of the hill -- where the chapel now sits -- from the main gate was like. Today the view is obstructed by trees and overgrowth above the Monument aux morts sculpture in the center of the photo frame.
this shows the avenue after it was dug out, probably c. 1920s