The reason they appear only on the 150 cards is because Knapp and Pancoast were finally granted a patent in Feb. 1910 that finally fixed a major defect in the lithographic printing presses at ALC.
The blue lines on the cards are called
gear streaks. They occur due to uneven pressure on the rubber rollers of the printing press.
The T206 cards were printed using a "web" (meaning paper roll) offset lithography press. This was new technology at the time, replacing the old sheet fed presses. Allowing for greater production numbers.
Because they were printed on an offset press, the cards never came in contact with the metal printing plate at all. The image was transferred to the card via rubber roller.
(In this simplified diagram, the ink gets transferred from the printing plate, to a rubber "blanket" roller, and then to the card.)
(CMYK printing for T-206. First the black border and name. Then yellow, cyan and magenta. Then the back in blue.)
The problem at the time was that the rubber rollers were made of pure rubber (instead of a harder, more durable synthetic material), which didn’t hold up well to high production numbers and tended malform, snagging the paper and changing thickness easily, causing uneven tension in the press, which would then lead to the paper moving out of position in a diagonal direction, shown below:
Basically a paper jam. One that would move in a circular path as it went around and around (based on what the printing machine probably looked like – a big circle). As the rubber would swell up slightly due to the pressue, it would make contact with the inked plate, causing the streak on the paper (and also scratching and damaging the plate).
In order to combat this, Knapp and Pancoast devised a fix by inserting a middle section of the web feed (highlighted in yellow above) that would control the tension and keep the paper pressure even most of the time. At least, that’s what it looks like based on the illustration and the explanation in the patent application.
Not sure exactly how the mechanism worked, but it had something to due with keeping the tension balanced with the fabric (I believe this was the term for the “blanket”, and I’m not sure if they’re referring to the rubber roller or a blanket wedged between the paper and the roller.)