That looks and measures much the same as mine. My understanding is that one kind is for the seatlug without a slot, and one for one with. This one came with a slot-less frame.
The thread is odd; it appears to be 1/4" x 1.0mm. A 7x1 die is a *very* loose fit, but a 6x1 won't even start.
It'll be a bit tricky to make due to the offset, and the seatpost seating-surface really needs to be cut accurately.