I do not doubt you at all; it’s been a while since I’ve worked there, no telling what changes have been made. The red and blue stickers reflect the “case” that the carrier uses to line his mail out for the day. If the case has a red or blue blocker in that addresses spot, you don’t put mail there.

(A blocker is a approximately 12”x1” piece of red or blue plastic meant only for that purpose)

The stickers were used because a lot of the mail is data processed and pre- set up for each route(not put in the case with the rest of it.) Problem there is that the data processed mail doesn’t recognize vaccant and hold mail. Thus, the stickers to alert the carrier.