USA Scott 2181A Overall Tag Vs 2181B Prephosphored Surface Tagging - Solid Appearance

I'm trying to differentiate 23c Mary Cassatt Great Americans definitive Scott 2181a overall tag vs 2181b prephosphored surface tagging - solid appearance. I have about 100 used copies that are not mottled or block tag, appear overall tagged but other than some having a brighter, more intense yellow color and darker glow I can't see any clear discernible difference. I concentrated on copies with selvage but couldn't find any clues there. Is using the darkness or intensity the way to go? Is one type scarce and I might not have both types? Neither variety has a value listed in my 2020 specialized.

Any tips? Thanks everyone.

I'm using my trusty old Raytech model LS-4CB, tried using both wavelengths.

There are multiple examples shown in the Stamp Smarter tagging database https://stampsmarter.org/features/TaggingView.html
Don

WOW! How did I miss that database? Thanks, Don!

I think I'll try comparing the paper colors next. The Stamp smarter database notes one paper looks grayish other brown.