This was discussed elsewhere as well, the front axle is forged, not cast!

This is a cast steel 42CrMo4 subjected to normalized and hardened at 35 HRC.

your front axle looks brilliant and a great effort
thanks for a reply to my question
certainly 42CrMo4 is a very good steel
this maybe twice the strength of original

Please do not be mistaken about the quality of the steels Molsheim used.
One of the favourite steels that Molsheim used was AD3 made by the famous French factory Aubert Duval. This company is still in business and still produces high quality steels for special applications.
AD3 is a low carbon case hardening steel that Molsheim used for front axles and steering arms etc. in unhardened condition and for a lot of other components in hardened condition like the rollerbearing crankshaft components. Do not underestimate the quality of these crank parts as I know several original crankshafts still running after more than 80 years.
A typical composition of AD3 is C=0.15%, 0.55% Mn, 0.8% Ni and 3.0Cr.
Strength is roughly equal to 42CrMo4 so most certainly not half as you suggested. 42CrMo4 is good choice for a cast axle but would be an unsuitable choice for the production of an hollow axle made in the traditional way because of the forging involved. The high carbon content means the steel is through hardening under air cooling and that is what you want to avoid as that happens multiple times during the forging process.
Bugwrench