The only photo references I have for Heyde are in the chapter in O'Brien's book, and unfortunately, these particular figures are not shown in any of the photos. However, that's a relatively limited selection of images.
I went to my uniform references. In "Military Uniforms of the World in Color", by Preben Kannik, image 391 depicts a Greek infantry private circa 1913 (during the Balkan Wars) and these figures look more or less like that figure. Remember that the French style had been copied by many other nations, prior to the disaster in 1870, and also remember that the Austrians used kepis, of a slightly different shape, and they were a model for some of the smaller states in the Balkans. I think these figures do represent Greek line infantry from the first decade of the 20th century.
Prost!
Brad