Only the GP cars have a hollow front axel ? What about the Royale ?

Hollow with a 40mm hole.
Bugwrench

T46 and T50 have hollow axles.

maybe all have hollow axels ?

no

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Vive la Marque !!

PLease be more specific. What is 'all'?
Bugwrench

T37 has a solid axel , so how are all the others ?

Making the axles hollow was (and still is) a matter of price. So the racers and luxury cars had hollow axles and the tourers and 'cheaper' racers just solid.
Just a shame it cannot be seen once it is mounted under a car...

are there any small holes drilled to check the wall thickness ?

I don’t think it was seen as necessary as they were forged not cast, on a GP the holes where the forging was closed over is a by product of the forging process and visible on the end of the king pin eye. Where there is a hole in this position that has clearly been drilled the axel is almost always a reproduction and if hollow probably cast.

There would be little point as the GP axles are machined both inside and out.You have to remember that the axle starts out as a straight forging.Boring through and then machining the center part of the outside means that the wall thickness must be correct.

And any out of line drilling is visible in the springboxes!
Bugwrench

some Molsheim drawings show 1,5 mm holes drilled in several places .
" TROU DE REPERE 1,5mm "

"some" please be specifique and say which drawings give a point as it may be mis read or mis understood.if presumably you know which ones.
accuracy in the question is important to avoid misunderstandings and innacuracies or annoyances in the answers.

Yes, but you forgot to mention that this is followed by: 1,5mm profondeur!
Be aware that a "trou de repère" should be considered as a professional alternative for the centerpoint marking.
Bugwrench