It is probably a Boeing weld process specification. Most likely proprietary data - they will not not likely share details publicly if you do not have a contract with them.
It could also designate a Boeing proprietary extrusion (extruded shape). They have their own numbering system for these. They're usually special shapes made for specific uses on their planes. I've seen them listed in aluminum product lists but they never show what they look like or their dimensions.
Even though they're proprietary, you can sometimes find pieces of things like this at Boeing Surplus up in Renton.
