These are similar to the colors discs or squares seen on a sheet of stamps that are issued by the postal department. For stamps it is referred to as 'Traffic Lights'. Collectors of stamps often buy a block of 4 or 6 stamps with the corner that has these color discs. If the basic color (C, cyan,M,magenta,Y,yellow,K,black) discs or squares are perfect, then it means that the image or material printed is aligned properly. If you see Inkjet printers with external ink tanks, this will also have the four basic colors, C,M,Y,K.
What you are referring to is something similar but on a large scale, seen at the corner of the sheet that is being printed. I think, it acts as a identification mark for the person supervising to see that the colors discs are clear and perfect, if they are blurred that means the images printed on that sheet also will be blurred. Since you've mentioned the time frame, it would be interesting to see whether this purpose is still needed in automated high volume printers.
Will try and post an image of some of my stamp sheets with the traffic lights once I get back home.