This one was a bit of a puzzle. I think I have it more or less straight now but confirmation would be much appreciated.
First, an image of the stamp which is a vertical pair with coil perforation. This makes it BW 402bc.
Just to confirm that it is 402 with Helecon dosed ink and not 401 with normal ink, here is a UV image of the fron (And yes, I need to improve my photographing by UV light.)
Just out of interest, I also viewed the rear under UV light, per the next image. It has a pink cast to it which could make it 402a. (BW 402 used up some Helecon paper stock, but printed on the non-Helecon side.) It is very similar to the reaction to KP5D paper, but not KP6T which is distinctly orange.
Finally, I checked it out for varieties and it looks as if the upper stamp could be BW 402d - retouch to four dotted lines in the top left corner. Now, on this issue I have some reservations: maybe the cliche for the upper stamp was rolled in a little heavier than for the lower stamp. Here is a comparison of the two Top Right Corners.
And finally, how would a combination of factors like this be expressed as a catalogue numbers: 402abcd or 402a/bc/d? And how would it be priced out? 402a in the 2019 ACSC is $25 per stamp, 402bc is $25 for the pair, 402d is $75 per stamp.
Your comments will be appreciated.