I have a block of four, so I took some pictures of different stamps under a microscope to see if it shows any better. I also adjusted some of the hues/contrast to better make it show. I think that I have the 915a with both rose and blue over the black.
The first picture is a closeup of the flag itself:
The second two are closeups of the bottom of the flag pole, trying to show that the blue is over the black:
And the last is a closeup of the top right of the flag, trying to show a slight bit of black from behind the red:
Looks like a 915a to me. What do you think?