The self-proclaimed "faithful slave" used to claim that the "anointed remnant" still on earth were doing this marking work.
w83 12/15 p. 17 par. 6 - "Today there is only a small remnant of the members of the “royal priesthood” yet in the flesh on earth, according to the world report from the celebration of the Lord’s Evening Meal on March 29, 1983. These serve together as the figurative linen-clad man, who does the marking of worthy ones in their foreheads."
In a very recent Watchtower they abandoned this pathetic doctrine and now give the credit of such marking to Christ himself!
WT June 2016, pp.16,17 Questions From Readers: "In the modern-day fulfillment, the man with the secretary’s inkhorn represents Jesus Christ, the one behind the scenes who marks those who will survive. The great crowd will receive their mark when they are judged as sheep during the great tribulation. This will put them in line to receive everlasting life here on earth."