Why does Boeing 247D have reverse slope windows when they were removed from the D models?
The Boeing 247D, DZ203 has construction number 1726 and was built in 1933.
The first production Boeing 247D produced made its first flight on September 5. 1934 and was scheduled to be delivered to United Air Lines late September 1934. (It didn't enter service with United immediately, instead it participated in the 1934 MacRobertson Race first, where it finished third.)
So DZ203 was built as a standard Boeing 247, and then later converted to the 247D standard. In the conversion the engines, cowlings and propellers were changed, but the windshield remained in its original style. This was common for 247D conversions.