I'd say weight and date. If it falls in the 1933-1939 range and is an upper rung Funk, I'd guess the frame to be of the Siemens-Martin steel which had a strength of 55-62 kg/mm^2 where the typical drilling steel was rated at 37-45 kg/mm^2. It may have been some skin depth treatment. And that would have been Emil Martin and son Pierre Emil Martin along with Carl(Charles) Wilhelm Siemens.

Kind Regards,

Raimey
rse