Most of the 3001old bricks with the mold number inside the tubes were made in white, red, blue, yellow, black and light gray. This is the first time light gray bricks show up in regular production. LEGO was keeping the amount of (expensive) color changes on the machines to a minumum, so I would not be surprised if many numbers can only be found in one or two colors.
It seems like only the bricks with numbers 343-358 were made transparent. For the transparent bricks LEGO to using a mold with the mold pip on the short side, all the others have the mold pip on the stud.
This numbering system started when LEGO was making bricks with thin walls and split tubes. LEGO was not really satisfied with that design so the next bricks were made similar to the thick walled previous design, but with some changes because the mold pip position moved to the studs.
The molds consisted of 8 positions. These were numbered using letters or numbers inside the stud holes.
[Original post and comments on Flickr.]