From FM23-6 M1917

"3. REAR SIGHT.-The rear sight of this rifle has no wind gage.
The leaf contains a peep sight which moves vertically on a
slide, and hence makes no correction for the drift of the
bullet. The battle sight is of the peep type and is attached
to the lower end of the leaf. It is raised to position for aiming
when the leaf is laid, and is adjusted for a range of 400 yards."

(I think) Ferris says it is actually 450yds though, and mentions the contradiction in his book.