Why does Boeing 247D have reverse slope windows when they were removed from the D models?

Question

Have a look at this article on a famed example of the 247D, DZ203, the first aircraft to make a completely blind landing. The first image is the aircraft during its flights with the RAF, which places it after 1942. Note the reverse-slope windows that are a hallmark of the early 247's.

However, this is a D model, and the D model of the 247 is noted as having had these removed and replaced with normal slope-down windows. This can be seen in the images of the other 247Ds on the same page, which were produced in the same batch of aircraft.

Does anyone know what's going on here? Were certain batches one way or the other?

NOTE: No, this is NOT a dup. The original answer from a while back states that all 247D's had forward slope windows, but this example clearly does not.

Answer 1

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.