I may be wong, but sort of remember the Lumo series of leads to be made of some sort of Plasticky material. Used them on Mylar, for lead pencil often smeared. I remember they were difficult to sharpen when in a coffee grinder. We had yellow and blue pencils. Blue would not show up on a blueprint, whereas yellow showed up with a force harder than black pencil. Had to use some sort of special white eraser to remove the lines or text too.
Here in California, we had the major blueprint places get rid of their Ozalid type machines and go to the dry OCE' styles about four years ago. Said the ammonia was hazardous, for operators were not disposing of it properly, but just dumping it out.
I don't like the new OCE' machines for Repro's as there is a gloss on older linen, and it makes for a speckled spot on the final paper, or will not let light penetrate on through, and thus pick up work done on BOTH side of a sheet. The newer machines also ignor colored pencils too.