1920 DWM question

[John Sabato]

Could you please expand a little about this "design change" ...like when did it take place, what was it's function and what models are likely to be affected?

The reason I ask is that to grind down and change this curved surface would affect the virtually the entire top of the receiver not just the area over the chamber without making the grinding more obvious since the curve blends into the relieved corners of the top outside of the upper receiver.