I would agree with you (and the engraving expert) that pantograph engraving is how the markings were produced.

My take on it is that prototypes are not marked unless it is deemed necessary, and this one probably lay around in its workshop for quite a while unmarked because the decision not to commercialise it was taken quite early on. At some point it was archived and so it was decided to mark it, just for the record. The easiest way to do this would have been with a pantograph/spinning cutter engraver, something that any large engineering enterprise would have to hand. The rather crude outlines to the lettering under magnification are consistent with such an engraver being used on steel, as they give much sharper results on softer metals like brass or aluminium.