I'm sure you have researched this Pierre but for the others following Borrani wire wheels used in racing were painted in lieu of chromed.
https://www.borrani.com/painted-borrani-wheel.html
Painted Borrani Wire Wheels
Borrani wheels do come in different finishes. Back in period if the wheels were built for racing they were painted. Simple as that. Faffing with Chrome was more effort, but also in the day there was always the risk of nitrogen embrittlement. What was critical for a racing car on Borrani wheels was strength, and mild steel spokes that are simply painted was what they did.