It's the Eagles' bye week, so we have the time to answer some of the crucial questions lost in the rush of regular season weeks. Today, it's the mystery of S 8, the giant characters that appear on the visible back side of coach Kelly's laminated play card.

The letters are huge, with a font size of about 244 points, so he must expect people to see and read them. The S is orange, and the 8 is green. What could it mean? A source inside the Eagles organization had no idea but admitted he was curious, too.

One theory is that it's a signal for players to see. Something about safeties? Could it have different meanings depending on whether the coach holds it upright, or sideways? Seems unlikely. S and 8 would be terrible choices for a visual signal since they look so similar.

In fact, different letters and numbers have been used since Kelly came to Philadelphia, and each set seems to last for several games. S 8 has been in use throughout the 2014 season, from the first preseason game on, but several combinations were used last year.

Kelly started his pro career in the 2013 Washington and San Diego games with a reversed T3 (white letters in a black block) over a black T1. By the time of the Denver game in week four, he had switched to a green J2 over a yellow J1, which repeated in the New York Giants game in week 8. The three games in between had J1 on top and J2 on the bottom.

And from the legendary Oakland game in week 9, which put Nick Foles in the Hall of Fame with 7 touchdowns, through the playoff game against the Saints, the code was S1 over S2, in the now-familiar green over yellow.

What about at Oregon? I don't remember seeing these big numbers back then and I can't find any in dozens of photos from that era, either. His playcards were your standard highly detailed lists, with a distinctive image on each -- one or more lucky shamrocks for the Irish-American coach.

At the University of New Hampshire, a source tells me, Kelly had the image of Clippy, the annoying Microsoft Office "helper," because he found it relaxing. (See? The mystery just keeps deepening.)

Whatever the meaning of the letters might be, the function of these pages was revealed by a single image from the 2013 San Diego game. The big numbers are simply a cover sheet, with a blank back, to cover the play call details from prying eyes. It's a third sheet that folds over the actual plays. Of course, this wasn't enough to stop ESPN from broadcasting a shot of the actual plays, but I'm glad he's at least trying.

As for the meaning, it's interesting that the combinations stick around for a few games at a time. You have to think it's a way to identify different play sheets, implying that the Eagles' offense changed for the Denver game. The much more successful S1 - S2 offense came in mid-season after the team settled on Nick Foles, and they went 7-1 the rest of the way before losing to New Orleans in the playoffs.

My guess is that these are model numbers, like the Mazda RX7. The 2014 Eagle S8 has not been the high-performance model people anticipated, due to problems in the front end, but as repairs continue it should be able to go from 0 to 60 yards in under 4 seconds.