On the photos I could find on google, all the later DG singles have a seat back mounted headrest, at least the 300/600/800 I found.
100/200/400 are the only ones I can find with canopy mount. It appears DG themselves changed it.
It’s only common sense to see no good is going to come from the headrest in a canopy ejection scenario.
I do want to have a headrest I’m going to contact DG about fitting a later model seat back, might as well get something for the blackmail contract.
When I talked to my CFI just a couple of months ago he was quite insistent that having no headrest was better from a lookout point of view.