Origin of the 'Picnic Table' Glyph

[Dobri]

Is that actually an attribute of the symbol? As far as I know, the only versions of x that have the vertical stem are those found on f57v, but that page also includes some without. 

Most (all?) of the other instances elsewhere in the VMS have no vertical stem, like these.

Unfortunately it may not be possible to date anything based on the invention of the symbol in any case, as it's a fairly easy symbol to draw and may have been independently invented many times, similar to symbols that look like o. 

For example, here's a similar looking symbol from a sculpture in "Kankali Tila" from the first century, which I doubt is related to the VMS:

A vertical stem could conceptually be an attribute of the glyph indeed but this reduces the possibility of the glyph being related to Eastern scripts which were accessible along the You are not allowed to view links. Register or Login to view. during the 15th century. 
It this case, it could be invented by the Author of the cypher manuscript, one option (among many) is to have a You are not allowed to view links. Register or Login to view. above a chevron, like in You are not allowed to view links. Register or Login to view.. 

The Kankali Tila inscriptions were written in the You are not allowed to view links. Register or Login to view. which is the ancestor to dozens of scripts used across South and South-East Asia. 
The 'picnic table' was not rediscovered at different times, it was always there as a glyph (with curvy legs and one of the legs being slightly longer than the other) but its meaning changed as languages evolved. 

The You are not allowed to view links. Register or Login to view. was introduced in the second half of the 13th century with a 'picnic table' glyph having for the first time straight legs of equal length and the same glyph was also used later (with a different meaning) in the middle of the 15th century in the You are not allowed to view links. Register or Login to view..