So it’s a mid fifties 600mph airspeed indicator that’s being exported to France or a French speaking nation.

So 600 mph equals jet.

In the mid fifties the France military used knots and it’s civil side was metric kph.
Pretty sure the Swiss were all Metric.
What did the Belgium’s use?

Sticking with the French theme, and assuming it would be fitted to a jet for export to a further country which still used mph;-Sud Aviation Caravelle gong to North America or maybe a Dassault Mystere going to Israel