I know the 9N12131 base with a circle 2 casting mark do not have the vents in them. The circle 3 bases do have two vents in them. Some have the part number cast into the some don't. So what is the the circle around a number trying to tell us? Is that a casting code or a revision? And how do I determine if they were ever made in cast steel and when did aluminum start? Would a blueprint of part number 9N12131 tell me what the base was made of?