I too, noticed the late date. [Personally, I don't subscribe to the conventional dating scheme of Egyptian history, but that is another issue beyond the scope of this thread.]The problem is, they havent build anthing worth mentioning since 600 B.C.
However, the merkhet in the museum and the picture are not assumed to mark the date of the invention. As it is a simple device, Egyptologists surmise that it was in use long before the 600 BC date. As stated by I.E.S. Edwards in The Pyramids of Egypt, (p257-9 Pelican Original Paperback):
Egyptian texts giving information on the method of orienting buildings are few in number, late in date, and formal in character.
[...]
The word merkhet means literally 'instrument of knowing' and may be translated as 'indicator.'
[...]
In spite of the relatively late date of the inscription referring to the episodes of the foundation ceremonies, there is no reason to doubt that they preserve an ancient tradition.Some indication that similar ceremonies were already current in the Pyramid Age is provided by a fragmentary relief found in the Vth Dynasty Sun-temple of Niussere, which shows the king and priestess impersonating the goddess Seshat, each holding a mallet and stake to which a measuring cord is attached.
highlights added
Also, the Egyptians would initiate the construction of a building with a ceremony called the "stretching of the cord," which is believed to be a means of orienting a building to the cardinal points. [Keep in mind that a magnetic compass does not point to the North (axis of rotation) Pole but rather to the Magnetic North.] This could have been done with either/or solar or stellar observations. Here is an interesting article on alignments of the Egyptian Pyramids:
http://www.world-mysteries.com/alignments/mpl_al4.htm
The question of the method followed in orienting the pyramids has been the object of a detailed study by Zbynek Zába. The documents prove beyond any doubt that the initial operation in erecting an important structure in Egypt was the ceremony of the ”stretching of the cord,” by which through the observation of stars with some sort of transit there was determined the North-South direction. The East-West direction was marked by tracing a perpendiculiar to the basic line.
Zába observes that since the pyramids were oriented to the North by the observation of stars, the position of the Pole must have been obtained by bisecting the angle formed by the two extreme positions of a circumpolar star. Most scholars are of the opinion that the orientation was obtained by this procedure or by a similar one. But a few scholars have observed that a much simpler method of obtaining the northerly direction is to observe the direction of the shortest shadow of the sun at the solstices.
nick c