This has so far been a great post and I hope it continues. It's true, nobody knows how the card was made. Even with all the proof in front of us, it's so hard to say it's 100% true. I keep forgetting that we are talking about T206's and anything is possible with them. I was 100% sure it was Polar Bear ink but then I put next to a Piedmont 150 and now I'm leaning more in that direction. As far as all the different Piedmont 150 or 350 shades of blue, all it take's is one shade to match for it to be a possibility. Now that would be a funny movie title for this card... 150 or 350 Shades of Blue.

As far as the scrap 150's with 350's on them, what if that's how they used scrap sheets. You print one side, proof it, make you adjustment and print again. But you wouldn't send that same sheet into the press. It would still be wet and make at offset mess of everything. You'd put it in a pile and use another sheet. Maybe the next day or next month you test print on the other side of those scrap sheets. That's how you would do it for the least amount of waste. Maybe they really cracked down on the waste at the job. Another unknown.

Where did you find or buy the card from?

I bought the card in 2015 but not sure exactly where but I didn't notice the blue ink until 2 years later in 2017 when searching for EPDG backs for print marks.