The Douglas DC-8 used leading-edge slots. Unlike slots in light planes, this one was door operated.
I thought Douglas must have patented it. But as far as I searched, they did not. So it must have been an older design. More researching later, I arrived at a patent, inconveniently titled "Airplane wing", by Frank R Maxwell (image below). Which is the same as the DC-8 minus the pop-out upper part. Earlier designs by Maxwell for this design date back to at least 1928.
And indeed a 1937 NACA report, Wind-tunnel tests of a Clark Y wing with 'Maxwell' leading-edge slots, uses the term Maxwell slot to describe them.
For a given slat size the Maxwell wing with the narrow slat has approximately the same aerodynamic characteristics as the Handley Page type of slotted wing.
Did any metal airplane use a true Maxwell slot? One example would suffice. I haven't been able to find any. I think the 'pop-up part' is important; the DC-8-style slot did not work on the DC-9 without vortilons (during development).