Unlike rxvt-2.6.4, 2.7.10 does not shift the NeXTSTEP scrollbar SB_BORDER_WIDTH pixels to the left when "scrollBar_right: false" is specified. This results in the black border around the scrollbar being drawn on the left, where the window manager's border already is.
I have attached what change I have made to simulate 2.6.4 behavior. It also includes additional XDrawLine()s to avoid the stipple pattern being visible in the gap between the scrollbar buttons.
I don't know whether this can actually have any priority at all. But I like the 2.6.4 way of displaying left-aligned NeXTSTEP scrollbars better.