It appears that the same steel is also referred to as Rchling Spezial Stahl R7L. Now Herman Roechling(1827-1910)-(Vlklinger Htte) developed a Spezial Steel in 1916 for the German Military hard-bonnets or helmets. Might be a stretch but this just might be the same steel?? But not sure if a helmet steel and tube steel would be similar.

Kind Regards,

Raimey
rse