Ted makes some interesting points, some more interesting than others.
The percentage of cards showing marks from plate scratches or streaks is currently fairly low. This could be from a few things. It could be that collectively we've only looked at/for them for a brief time. Pat has pointed out a couple of the other limiting things. In comparison to the P150's there are fewer scratches so fewer cards from a sheet will be affected. And when the scratch happened will have a lot of bearing on the percentage found. I think it's simply too early to draw much of a conclusion from that percentage.
The question of what sort of press produced the set is a good one.
If they were produced on a then fairly cutting edge rotary offset press that used metal plates it opens up a lot of complications. The plates at the time were expensive and not simple to produce.
http://sites.tech.uh.edu/digitalmedi...y_of_Litho.pdf
I don't see a mention of them being saved for reuse, but it's possible they may have been as it was a fairly common thing with the stones. (Some were saved, others were resurfaced for reuse)
If the plates were saved, they would be just as likely to be damaged as a stone. That damage might be different, as different accidents happen to large, inflexible heavy things than happen to light flexible things that happen to also be large.
Diagonal streaks aren't all that likely on a rotary press. And consistent diagonal streaks are even less likely. Streaks parallel to the direction the sheet travels are likely, but these marks are not parallel to either a sheet run sideways or vertically. And a diagonal layout for rectangular objects would be really odd.
Rotary offset plates can get scratched, just as stones can. I have a 1981 Fleer card with a nice red line from a plate scratch, and I'm very sure a rotary offset press was used.
So to some extent the type of press used and how the printer handled the plates does matter. If plates weren't typically saved, consistent diagonal marks on series separated by time would most likely indicate a stone rather than plates.
Another possibility would be the printing of various groups of fronts in different quantities either at the same time, or consecutively - group 1 Monday and Tuesday, group 2 Wednesday........with the backs printed last. I think that's unlikely, especially as one group typically shows flaws from dry or worn plates. (Using plates to include stones for simplicity)
Even if the marks are indeed streaks, they're consistent enough to give us an idea of what cards were next to each other.
There are fairly consistent streaks on more modern cards, once the rate the sheets were fed at got high enough static electricity became a problem, and the solution was dragging a string much like tinsel along the sheets being fed into the press. With proper drying in between colors, they shouldn't happen, but on a lot of 50's era cards they're common.
I don't think the presses of the era, even rotary ones had a high enough rate. But some part of the press being loose might cause a streak.
Personally I believe these are too consistent to be anything other than plate damage. Scratches most likely, but if it was a rotary press they could also be cracks, which would explain why they're less common.
All in all, they're worth studying.
Steve B