Yep, mine is Stirrup brand, too.
Henri, I don't know what sorts of supply shops you have in the Netherlands. I think, from examining my intact hammer, that the nail holding arrangement on that design is two balls, two springs, and a screw that retains them. In the U.S., the springs could probably be cut from a stock spring available at any hardware store; and a bearing house could supply the balls. The tricky part would be finding a suitable screw to hold it all together, particularly for a German hammer, in which we can assume the screw would be metric. You'll have an easier time finding a metric screw, and I would think a standard short setscrew* (as used to hold pulleys on shafts) might do the job, retained by some thread locking compound.
Loading in the parts from the threaded hole side, it would be spring/ball/ball/spring/screw. You'd have to experiment to find the right spring and the right length of spring. You want the balls to touch firmly, but be fairly easy to move by shoving a nail past them.
I'd suggest you drop by and we could look yours over, compare it to mine, and we could figure out how to fix your hammers; but I'm afraid the drive might not be worth it from Bussum (I looked Bussum up on Google Maps; pretty little town).
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*The British term is "grub screw," and that may have traveled over the Channel as the standard term.
