I got a blacklight to use with my stamp collection. And being a kid with a new toy I spent a while wrecking my eyes using the blacklight on nearly everything in the house the first night I had it. I had a monster box of 91's open on the desk and spotted it. I figured they should all be the same, but under UV the box looked like a huge bar code.
Both light and dark logos have the difference.
There's a third sort that shows as a very dark red, but it's very hard to spot.
Also there's a few with a cardboard stock that is mildly reactive to UV. Both of those are far less common.
And to top it all off I've found 4-5 out of about 10,000 that the glosscoat reacts by turning greenish.
I think most companies in the late 80's-early 90's were using multiple printing plants, so you'll find all sorts of little things in those sets.
I have a list of the variations I've found in the 91's somewhere. It's written, so I'll have to make a spreadsheet someday.
I keep some lists of the oddities I find, I just haven't brought them all together into one list.
Steve B
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