Huntinguy is right, the darker , the less dense the material is. Numbers are lead, usually placed to indicate direction and or distance. If using x-ray to judge mat thickness, quite often a lead "stair" is placed on the film and shades of grey from it can be compared to the object being examined. There is definately a "gap" in the splice, I see three pieces of material. The 0 part, the 1 part and the spigot attached to the 1 part. The dots and "cracks" I believe are pits and /or erosion on the outer surface. An oblique shot could provide more needed info as well as a normal picture. With what we`re given, I would say that there is very little structural integrity to the joints left (rear) side, definately more information would benefit analysis.... as a disclaimer, I am not professionally trained in radiology but have viewed x-ray/ performed lots of welding QC over the last 30+ years... my 2bits Kerry