What you are confusing is that dividing the 7 part cubit into 6ths did not contitute a different measurement system any more than dividing a 6" rule into 5th does.
The Cubit Legin proposed for the Canon was 3 Royal Cubut of 7 parts from floor to hariline.
Each cubit in height was then divided into 6ths, but this did not constitute a different measurement system.
As for the glyph for Cubit being a short Cubit, well it's not, because the glyph for 6 palms is a forearm and the small bird.
The 45 or 43 cm measures were a thing used on the Levant, and not Egypt.
Dave Light.
