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Because of where the markings are placed, and the fact they are an unrelated Spanish indicia, it seems clear to me they are simply obliterations to void/cancel the expired stock certificate, and not having anything to do with any actual revenue or tax purposes.
$50,000 is a lot of money, especially in 1906, so there are so many of the markings, and different colors, to make it very clear the certificate is all used up, and for whatever reason the clerk didn't have a rubber stamp that said Void or Cancelled.
Before these pólizas were made, the stamps were applied to the document by government institutions in provinces and accounts were kept in a way the these stamps could be proven to be genuine. In the mid 19th century, they were applied to adhesive labels, bij the Fábrica de Sello in the 1893 merged into de Fábrica Nacional de Moneda y Timbre. In 1906 and later, they were used on adhesive labels.
If the FNMT did not use an old document to test the stamp (I cannot imagine why they would), these, almost certainly, are forgeries. Maybe FNMT sold the devices after the póliza was abolished in the 1960s and someone had some fun. These are not stamps that someone at a post office or local tax office, or clerk at some administration had available to cancel a document.
The impuesto de timbre could be related to the gold bond being transferred to someone in Spain and stamp duty was raised. Then the document must have been sent to the FNMT for the stamps (pólizas) to be applied directly.