Oh, yes: it is a 16t. The chain gets pretty close to the bolt heads, so you ceratinly can't drill a 14t. Maybe a 15, depending on what chain you use and how large the bolt heads are.

Also, DX cogs are surprisingly thin. This is something like 2.2 mm, while my track cog was exactly 3mm thick. Maybe it's actually not a 1/8? Dunno.