I'm having trouble understanding part of the logic surrounding a section of a paper I'm reading (fly developmental neuro/partially a genetics question), anyone able to help out?

The paper is here: http://www.pnas.org/content/108/18/7571.long

or here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3088640/

Basically I'm confused by the fourth results section and the logic behind the first experiment they mention in this. They talk about generating clones of the mutant eye discs.

I understand from figures B-B" what they mean by "mutant clones in the most posterior region of the eye disk were infrequent." I think that I understand the basic gist of the the procedure they used to produce eye discs which were fully homozygous.

I'm having a hard time understanding why they did this and how this hones on the defect specifically being in R cells as opposed to "in other cells in the optic lobe."

Perhaps I'm misunderstanding the techniques more than I realize? The only discernible difference I can understand here is that the eye discs of the second flies are homozygous KOs across the entirety of the disc, whereas the first have a lack of posterior-located homozygous KOs.

How does this show R-cell specificity?