I'd have to disagree... rearsets usually imply something aftermarket, as in setting the pegs further rearward than they came stock. Since these look like they have factory info stickers, that kinda rules that out. (plus they look too heavy for rearsets.)
They look more like footpeg brackets. Not being that familiar with what the parts look like for the 81 CB750F, I couldn't say if they were the rider's set or the passenger's though.