The chart has at 600m a pressure of 980 mmHg pression d'admission. Extrapolating it would be at 990 mmHg at 0m. The speed is clearly close to the 480 kph when extrapolating the chart to 0m, perhaps at worst 478 kph.

5 kph velocity loss seems a bit extensive with respect to the new windscreen.

They even did not measure any difference with guns installed or not (so including the effect that for level flight higher angle of attack would have to be chosen due to increased weight of the guns).

Pression d'admission translates into inlet pressure or manifold pressure so I think this is ATA setting they used.

http://dictionary.reverso.net/french...'admission