Received a SP31UV with accessories from AmazonUS for $18.49 (before sales tax) using the generous 50% discount code offered by Sofirn earlier in this thread and the build quality fit and finish are excellent.
Forum member Serlite's review was spot on and I received exactly what I was expecting which is a light that I do not really need, but the price was right for adding it to my collection for showing others and making comparisons. It's throwy bright UV hotspot will be useful if I ever need to cure UV-activated glue and in the meantime it will compliment the floody soft hotspot of my Wurkkos WK30 which also has a 365 nm UV LED along with both a white and red LED.
Sofirn's AmazonUS listing indicates the SP31UV is "Equipped with black filter optical lens which makes the ultraviolet light purer." I am not sure if the "black filter optical lens" is supposed to be noticeable or have a visually different appearance to the naked eye (as viewed when the light is OFF to avoid damaging your eyes from UV light), however the lens looks like plain crystal clear glass to me and I do not believe I am seeing any beneficial effect from this "black filter" since when the light is ON at either brightness level, the non-UV visible light almost seems to overwhelm the UV light portion of the LED.
On plain white printer paper the tint of the two lights appear virtually identical to me but sometimes there is an obvious UV light spectrum difference between the 365 nm UV LEDs of the SP31UV and Wurkkos WK30 which could be caused by the "black filter optical lens" of the SP31UV or more likely from physical differences between the two LEDs and this is most noticeable when examining white chicken eggs. The white eggs fluoresce and appear to be a "golden yellow" color using the SP31UV but are a very different "brownish orange" color when using the UV LED of the WK30.
Besides the color difference when examining eggs, every object I tested that fluoresced with the UV LED on the WK30 could also be noticed with the SP31UV, sometimes standing out less and sometimes standing out more, but I could not identify any patterns or scientifically understand exactly why these differences occurred.
The visual patterns on the two LEDs look different using 10x magnification. The 365 nm UV LED in the SP31UV looks sort of like a "pin cushion checker board" with 23 pin prick holes (four rows of five holes then one row of three holes where this particular row does not have a hole at each end of the row and is also one of the four sides of the LED and there appear to be thin wire electrical connections to the two corners of the LED where the holes are missing).
The 365 nm UV LED in the WK30 looks sort of like a "crown" or in ASCII representation as a |V| except the sides slope out like this \ / and the V is shorter like this v but wide enough to connect the top ends of the two sides and the bottom of the v does not touch the bottom line. All the lines are connected at their corners but there are two small gaps, one gap is at the point on the bottom of the V and the other gap is in the middle of the bottom line. The electrical connections to the LED appear to be at the two top corners of the V.
If I could have only one of these two UV lights, for me it would be the WK30 because it functions more suitably for my casual playful use. Neither of the two brightness levels of the WK30 UV LED are as bright as the corresponding brightness levels of the SP31UV but the UV tint of the WK30 UV LED contains proportionately less visible white light and being more floody with a less defined hotspot just makes for a more satisfying experience to psychologically (not physiologically) "blow unsuspecting people's eyeballs out" when using the UV light to examine the normally invisible security features on money, driver licenses, passports, various minerals and the incredibly pervasive normally invisible stained interiors of their homes.