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https://www.metmuseum.org/art/collection/search/21934

Charles V's spada da lato (c. 1550), now in the MET, features a rare sight in Iberian and German/Norse sideswords and rapiers I came across: a knuckle bow.

Arms and Armor attempted to explain the arrival of early knuckle bows here, but while it's clear the earliest knuckle bows appeared in falchion-esque type weapons, it's adoption in straight swords seens to be ... geographically issued.

The earliest "true early rapier" I could ever find is Gustav Vasa's Rapier, dated c. 1550 and made either in the Swedish Workshop or in the German Workshop that built the former. One of it's interesting aspects is how it doesn't feature a knuckle bow, something that I also can't recall in the Portuguese and Spanish spada da lato's and rapiers of that time.

But, as Arms and Armor pointed, there's a number of straight swords with knuckle bows in Italy or of Italian origins already by the late 15th century. Do you guys think it makes sense to conclude, at least for the 1480-1560/80, that Iberian, German and who else more either didn't do them, or rarely did them?

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Spanish rapier from c. 1620, at the MET
https://www.metmuseum.org/art/collection/search/25058

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Italian spada da lato from c. 1540, note the schiavona-styled guard. I suspect knuckle bows might have some slavic influence, which might explain why Venetian swords are so proeminent in knuckle-bow guards.

An Italian or Slavic origin of the knuckle bow would make sense. Single edged weapons based off of work tools with a knuckle bow coming from the tang are associated with Italy and Bohemia. I want to say that I read a source that said the Venice was a source of Messers in Germany.

Here is at least one German straight sword dated 1530-1560: https://skd-online-collection.skd.museum/Details/Index/287682

Interestingly, the Wallace collection has two swords that are attributed to the same maker, one has a knuckle bow, the other doesn't. So it is possible that in Germany both styles were popular, whereas in other lands knuckle bows became the rule.

https://wallacelive.wallacecollection.org:443/eMP/eMuseumPlus?service=ExternalInterface&module=collection&objectId=60973&viewType=detailView
https://wallacelive.wallacecollection.org:443/eMP/eMuseumPlus?service=ExternalInterface&module=collection&objectId=60974&viewType=detailView