i had lots of those t-handles back in the day, and almost all of the universal "fitzall" handles with the adapter bushings were made with flat threads as your photo shows.
if you are worried about the threads, use some PINK low strength thread locker on the adapter of choice, you won't have any problem.

the most common thread sizes back in the day was 5/16-18 and 24, and 3/8-16 and 24 [fine and course pitch] for most manufacturers shifter handles.
the hurst handles were usually 3/8-24 if i remember correctly. these handles had normal looking threads.
whenever those t-handles were introduced to fit everything, i remember them having 1/2-13 threads, and adapters to fit the 3/8 and 5/16" handles. later, the thread pitch in the handle grew larger, but i don't think it ever got bigger than 3/4".
if it indeed has metric threads, i believe it has to be somewhat recently made. within the last 15 years or so ?

in any event, i wouldn't sweat the threads you have. just obtain the adapter you need and go.