During upgrading and cleaning the forum, I found some attachments with file name started by "_" (underscore) instead of "R". This is quite funny, because my camera saves the photos with exactly the same character. I can even tell you when exactly it started to behave like that. Simply, one photo has been saved as R0015583.JPG and next one as _0015584.JPG. Now all photos since _0015584.JPG are saved with "Low Line" instead of "R" (the same apply for DNG).
I believe there is something wrong in GRDII firmware, causing such strange file naming. Please add your comment in case your camera does the same and if you can track down the number of photo, when it started to save photos in such strange way. Thanks!