Hi ulfberth,
It seems as you don't mind having this thread hijacked with further considerations on the subject.
Perhaps the following text, roughly translated from a Spaniard's post, would fix some possible failures on what has been considered so far.
http://mundomilitaria.es/foro/index.php?topic=11902.0
"These daggers were popularly known as Vizcaínas, due to the origin of a great number of them been forged with steel of Vizcaia. Together with the Roperas (rapiers) were carried those known as dagas de mano izquierda (left hand daggers) or, better saying, dagas de detener (parrying daggers), which was the name they received in Spain, once the ‘left hand’ term was a galicism provenant from the French denomination for this type of weapons, ‘main gauche’. The proliferation of renaissance fencing manuals concerning the development of combat with double weapons, took sword smith masters to design various types of daggers for such type of fencing. Therefore the appearance of the uniquely Spanish (Españolisimas) sail guard daggers, which were and are a symbol of parrying daggers; also other designs were developed, like trident daggers and rompe-puntas (sword breakers)
The left hand dagger had its origin in the XVI century, when the ‘double arms’ fencing appeared, that is, the combined use of the sword and left hand dagger for the parrying of the adversary blows and, being the case, take advantage of some mistake from his defense and penetrate in his body.
However its real expansion was known as from the first third of the XVII century, having been created in Spain its own version of this type of daggers combined with its companion sword, when pieces were made with the finest decorations.
The origin of its name is based on its hand guard, similar to a latin sail. This addition meant a notable advance in the protection of the hand, once until then these daggers only counted with a cross guard similar to their larger sisters for the parrying of the adversary blows, as well as to avoid the hand slide to the blade when stabbing.
Cncerning quality, they were celebrated in all Europe, not only those from Toledo, but also those from Vizcaia and Guipuzcoa".
If you don't mind, i will also include pictures of my own sail guard dagger.
Remember that, during the discussed period, Portugal was under Spanish domination, so not surprising that swords and daggers were alike in both nations.
Also some clips on the art of fighting with these weapons may be useful ... skiping the language difference:
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https://www.youtube.com/watch?v=PyU5ndSTWxc
https://www.youtube.com/watch?v=jPLyHzaI-4I
https://www.youtube.com/watch?v=r1ovV5shSBk
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