Recently I was asked for an opinion on how to distinguish Scott 3980 from 3980a. Scott lists 3980 as overall tagged and 3980a as Prephosphored coated paper with surface tagging showing a solid appearance. I found the 3980a description confusing myself as both varieties are overall tagged.
The questions was whether the 3980 was the one that glows white or green. This question can also be confusing since neither glows "white" under SW UV assuming your UV source is properly filtered. With appropriate filtering, one appears bluish green under SW UV and the other a bright green/yellowish green under SW UV. Under LW UV one does appear whitish. Below are the two examples under SW UV.
The first impression some have is that the one that appears green under SW UV must be the overall tagged one since many overall tagged stamps appear this way. From the images above you can see both are tagged overall i.e. there is taggant covering the printed image.
The question is how to determine the difference. I examined the two under 100x magnification with a SW UV light source. Under magnification it is clear in my opinion which is 3980 and which is 3980a. Note images are somewhat blurry since the dept of field is so shallow and this also challenges a camera's auto focus.
Based on examining the diecut area under SW UV, 3980 is the bluish green example and 3980a is the green/yellowish green example. Under LW UV, 3980 is the one that glows "whitish". Hopefully people find this information helpful.