Did any metal airplane use a Maxwell slot?
Finally! The Vought OS2U Kingfisher (first flight 1938). At least the OS2U-2 model...

Production variant with minor equipment changes and powered by a 450 hp (336 kW) Pratt &amp; Whitney R-985-50, 158 built. [emphasis mine]

... had default Maxwell slots, thanks to a wartime NACA report.[1] The report makes no mention of the plane's model, but another report does in the references section.[2]
I thought, it can't be true, it must have been a NACA modification. But no, the first reference explicitly lists the modifications done. The first of which is what I needed:

Modification 1.– Auxiliary slats were mounted ahead and above the normal leading edge of the wing root, overlapping the inboard end of the original Maxwell slot arrangement by about an inch (fig. 11). [happy-emphasis mine]

And that's not all. The PDF is a 90 MB color scan, so, photos, naturally:

Photos are hard to come by, but I found one that shows the popped-up leading-edge from behind:


Source: ww2db.com; the plane was available in land/sea configurations.


1: Adams, Betty. &quot;Flight Measurements of the Effects of a Wing Leading-edge Slot and Other Modifications on the Stability, Maximum Lift, and High Speed of an Observation Airplane.&quot; (1943). https://ntrs.nasa.gov/citations/19930093568
2: Adams, Betty. &quot;Flight measurements of the effect of various amounts of aileron droop on the low-speed lateral-control characteristics of an observation airplane.&quot; (1943). https://ntrs.nasa.gov/citations/19930092611