The production photos seem to support the notion that the prop, as originally constructed, had a flat-head screw instead of the Weaver knob.
WOW! Those '81 photos are frakking UNBELIEVABLE.
I restored the 1981 photos a little... I left the "Original Prop Blog" logo on them to identify whose pics they are. If that is not all right let me know and I'll take them down.
I also notice that there are no white wires running to the site rod. Very interesting.
Though the white wire does show in the film, correct? Interesting indeed!