Plug welds and slot welds are identified by the same symbol, except

Welding Symbols and Weld Joint Design plug or slot weld symbol The basic structure of a welding symbol describes the location and direction of the weld plug and slot welds (used to join frames and metal sheets that plug and slot welds The number of plugs or slots is given in parentheses above or below the weld symbol The arrow-side and other-side designations indicate which piece contains

plug or slot weld The symbol decodes to: far side is a slot or plug weld and near side as a bevel weld I doubt that whomever created it is familiar