A little more insight based on my experience...
This lettering was "MOST LIKELY" placed on the tubes using a stencil, punched out of oiled cardboard. with a "MARSH" stencil punch machine. These have been used since WWII in all kinds of industries.
The stencil was probably held by hand while either being daubed with a Marsh Stencil Brush or sprayed with a rattle can.
Now... I have hand-drawn this MARSH font alphabet, using AutoCAD, from actual punched letters from a Marsh Machine. I do a lot of marking decals for military aircraft restorers such as Stearmans. I'm going to gather some pics together and post them. If it is concluded by the experts here, that this is the right font, I can create a stick on, removable stencil for spraying or daubing, if I have the height and specific numbers you need.
Note: If the driveshaft markings were "SPRAYED," the characters will have a fuzzy edge. My sticky stencils will not create that fuzzy edge. If the driveshaft tubes were marked with a daubing brush, my sticky stencils will create a closer representation.
Stay tuned for the "Rest of The Story."
Here's the Marsh Machine and Stencil Brush.