This valve stem looks to be like yours. It is actually of metric sizing, nominally called 6mm, actually about 6.1mm by my oriental digital calipers of unsure accuracy. The threads at top of stem appear to be metric as well, but the valve body seems to be threaded 1/2" NPT to fit our domestic cylinders of that era. The threaded seat is probably metric, and a bit smaller than our usuals.
If your Nemrod is also made in Spain, as this is, then I'd bet the stem would be a fit. It is straight and true to the eye when being slowly rotated in my drill press.