Subassembly combination for mail processing machines, including a letter feeding machine, and a moistening device having a rotatable drum for moving mail-wetting liquid and a sealing device being mountable on the letter feeding machine, the letter feeding machine having a transport cylinder for transporting mail, a common drive gear for the letter feeding machine, moistening device and sealing device, and a common shaft on which the transport cylinder and drive gear are mounted, and a serrated belt disposed on the common drive gear for selectively driving the sealing device and the rotating drum.