Origin of the 'Picnic Table' Glyph

[eggyk]

What makes the x glyph unique is the vertical stem above the chevron so that the glyph looks like a vertical arrangement of Τ (Tau) above Λ (Lambda).

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: