Last year, JFK researcher extraordinaire Roy Schaeffer alerted me to this one. Here is a collage that was created for my book, which traces the correct tint line of the windshield versus other areas of obfuscation in the Altgens6 photograph. The faces of Greer and Kellerman are not visible, even though, in the Queen Mary, which was right behind the JFK limo, Kinney’s and Roberts’s are:
Roy pointed out that Secret Service Agents Greer and Hill had distinctive white envelopes in the left breast pocket of their suit coats, both clearly seen here in the Sisco photograph. 
In this Towner film image iso, the envelope is clearly visible above the door line of the limo, as it makes the turn onto Elm Street.
When we compare with an Altgens6 iso of Greer, Schaeffer noticed that the envelope was in such a position that revealed Greer was turned around, looking at JFK. Here is a side-by-side comparison.
In this next image, we have outlined the envelope, which is faintly visible:
So, in rendering our Z255-Altgens6 scene in Blender, we obviously had to turn Greer around, in this manner:
The reactions of both Kellerman and Greer had to be hidden in the Altgens6. Those who have studied the Zapruder film have noted that Greer turned around several times, at 281-296 and at 305-313, the fatal shot. To that we can now add that Greer turned around much earlier as well, at Z255, where he is not visible in the Zapruder film, but is faintly visible in the Altgens6, a detail noted thanks to Roy’s keen eye.