It's not from a sheet edge.
I've only really done the first series so far, and I believe that sort of line is some kind of cutting guideline. Some are white like that, some are black or gray.
Nearly all cards in series 1 have a line of some sort, many have multiple different lines.
There are also cards that come both with and without the line.

I've seen the line on some others, but the uncut sheets I've seen only had printing marks along the four edges, not between every row. I'd always assumed these were the cards on the border, and the cutting mark was only placed there. If it was under every row, any card even slightly off-center top/bottom would show this line, looking at where it is placed on this card?

Yes, top and bottom. It may not be very card, but it's pretty close. I need to either find my list or rewrite it so I can post it. I was working on the first series for a while, but didn't really keep after it.

I don’t doubt they can be found on near every card. If the line is between every row on the sheet, then any copy of any card that is off center top to bottom would show the line. Most top to bottom off center cards don’t show the line however, so I think this line was not on all the sheets between every row. Otherwise, all these off center cards would have them in the same location

That is correct. They usually don't show on well centered cards. And they aren't on every sheet. And the line can be different, so it changed at times. (I also had consistent copies of several of the lines, so if there are two different lines that's a genuine difference.

Not necessarily a variation, but interesting look due to the lack of black on this pass.

Now that Bevans is the real deal. Blackless is beautiful.

http://photos.imageevent.com/exhibit...0Blackless.png

With all due respect, guys, a stray print booger here or there is amateur stuff. Let's see some f***ups!

http://photos.imageevent.com/exhibit...20misprint.jpg

Not mine, I wish it was. ETA, someone posted it on CU a few years ago, said he pulled it out of a pack in 1970 and has it to this day.

This one is tame compared to those two.

At least we know who those two guys are :)