de Lubicz may have been onto something a few of us have already considered also. If the meter value is set to 39.375 ins. the arc to chord ratio in Royal Cubits becomes this familiar value once again. 39.375 x 22/7 = 123.75 / 6 = 20.625 ins. The problem is as always, the modern meter obscures the fine details that makes any coherence of compatible units from other cultures impossible to reconcile. Oh well! The difference between the modern meter and the original Polar Circ. unit I am using is 1/8025th., which isn't much on small scale observations, but obscures large scale items such as the Earth itself.
The remen unit you mentioned of .518m, is too close to the Nippur Cubit value I regard as intended at .5184m and is in ratio to the full extent of the .525m RC value that Stecchini proposed by the geodetic Polar Meridian Circ., when multiplied by 12960000 arc secs. Your mentioning this particular unit was the first I'd heard of a similar unit in Egypt also, but what other uses could it have possibly been constructed for I wonder. Music maybe? LOL That might not be a joke.