Mine was slotted
but a quick search "Honda D shaped mixture adjustment screw tool" came up with several sockets to do the job..
Also instructions to make your own..
1. Get a 2-3" piece of 1/4" o.d. (1/8" i.d.) soft copper.
2. Drill the inside of one end out with a 3/16" drill. This will fit perfectly around the carb idle adjuster, but won't turn it.
3. Put the drilled out end against a vise or the garage floor and lightly hit it with a hammer until one side is flat and you get the D shape. Be careful, that copper is soft.
Found at https://www.triumphrat.net/threads/h...t-works.50259/