My opinion is probably not worth much because whenever I look at the photo of Luke holding the blaster, I've always seen two separate parts.
To me, it looks like the top "nut" fits on top of a smaller diameter washer. To me there is no split between knurled and straight sides. To me it looks like a rounded nut with knurling from top to bottom.
Something like this:
View attachment 530849
View attachment 530850
It has always seemed to me that when building a prop, you would choose to use what was available before machining a custom nut. Since these exist already (Rounded Thumb nut), it seems like they would have just grabbed one of these from the hardware drawer.
Try as I might, I just can't convince myself that I am seeing a single nut, with straight sides and angled top. So, I've pretty much kept out of the conversation because others don't seem to see what I'm seeing.