^
Ha, only thing clear about the space trooper lore, is that its clearly unclear! Everything from the number of actors, backpack design and the so called short forearm armor. From what I've read and the half a dozen photos from that days filming, there is no definitive proof of anything.
For my money though, zooming in on the image of the trooper with Lucas (which is presumably Johnston on set), the full shape of armor is visible on both arms with some kind of reflective black sleeve covering the bottom of the armor. Based upon the material and length of the original trooper glove, it looks clear to me that that black area is the glove stretched over the bottom of the armor.
That's my deduction anyway!
There's plenty of this going around on the RPF, Prop Den, and round tables all over the world
I for one enjoy the mystery. So much in SW (since the OT) is spelled out for us these days.
Cheers
Ahab