According to Wayne Neyens (in the Pinball Compendium 1930s to 1960s http://www.schifferbooks.com/the-pinball-compendium-1930s-1960s-1398.html )
the change was made in 1964 after his daughter was cut when playing on top of a machine and the glass broke.
You can bet before I get my Gigi operational again I'll make sure I have tempered glass in it.