re the "crease", I had posted a similar issue I had a few years ago, just tried doing a search and cant find it. Basically I had a rubbing issue on one side that I thought was the crease, to the point that I was wondering about either using sandpaper or a scalpel to trim down the "crease". In the end Im glad I was patient and thought things through, as the problem was simply seat position.
It started when I had made an angle adjustment, but when the bolt was loose, the seat moved forward or backwards a bit, and because there are no painted lines on the rails, I didnt put it back properly in the same position as before. Im pretty certain the seat was then further back than before, and so on my drop bar bike that it was on at the time, my position was a bit too forward on the seat. When I moved it forward slightly it took care of the issue right away as it moved me a bit more back onto the main part of the seat, removing the rubbing.
My case also was on the right side, but as Machka says, a little difference isnt necessarily the problem, and I suspect that it could easily be because we arent exactly symmetric so the seat "goes with the wind" so to speak.
I since put that B17 onto another bike when I bought a used B17, and on either bike I havent had that specific issue come up again--**I did learn to stick some white tape on the rails where the seatpost clamp thingee is, as a reference for "fore-aft" position. I lent the first B17 to my wife for a few weeks last year and thanks to the tape, was able to put it back on my bike and get it positioned properly.
From my experience with these two B17s, I would concur with what people say about the importance of being attune to small changes in seat position of angle and fore/aft. I think you have be more observant with leather saddles given that they do change shape, especially early on like yours, and slight changes can make a real difference in long riding comfort.
Make some small changes, but especially be observant of keeping track of the seats position as you change things, just so you have a proper reference (using tape on rails, using a bubble level)
you say you have seat set at level, a slight tilt back can help too (but I would suggest making one adjustment at a time, ie fore aft first, then angle, just so you can feel the difference of ONE adjustment and know what it does)
again, small adjustments are the key