I managed to solve an issue with my LPL 4550XL today. I figured I would post here in case someone has the same problem in the future. I was getting an inch long mark on some of my prints as shown in the photo. It was only happening with 4x5 enlargements so at first I thought it might be something with my 135mm lens but I couldn't see anything. I decided to pull apart the head and there was a black mark on the white plastic on the bottom of the mixing box. Maybe a scuff mark of some kind. It was near the edge so not affecting smaller negatives. I buffed it for 30s with a cloth and it went away. Just finished some prints and all is good.