The Sunset brass tenders I have all have KD #5 couplers in KD boxes. One is held on with a 2-56 screw in the center hole and the others are held on with smaller screws 00-90 I think but they could be 1 mm on the sides.
However, if you double head with another DC brass loco, though, you might look into the insulated drawbar Kadees (20 series) to avoid a short if the tender was coupled to the front of the other loco.
The mounting hole most likely came with the correct screw, but was lost along the way. Pete’s covered the most likely possibilities, other than it might be 00-80. They tend to be metric in more recent runs of brass.
The folks at Sunset might at least be able to tell you what size screws to look for, even if they do not have any to sell you. The Greenway outfit really specializes in spare parts for brass locomotives and they have a supply of scress.
Mounting a Kadee coupler and box on a brass engine tender is one of the few times you actually see those two side holes be used for mounting purposes – most guys just use the center hole. I do not have a Sunset tender before me as I type this, but for most engines the three holes are both drilled and tapped as they come from the importer.