Japanese do not eat various types of meat as European do. Therefore you will find less types of meat at supermarkets in Japan than in Europe. Beef, pork and chicken are common but for example duck is rare to find at normal supermarkets.
On the other hand, Kamonamban; duck soba noodle is quite popular in Japan. Soba noodle is made from buckwheat with very long tradition but soba noodle with hot soup appeared in the middle of 17th century. At that time, eating beef nor pork was uncommon in Japan but duck was quite normal to eat.
Kamo means duck and namban originally meant indigenous people living in the south, but from 16th century it means Portugal and Spain. When Portugal and Spain sent their missionaries in 16th century, they came from the south. At that time, Japan had no idea where these countries are located.
So Kamonamban came from Portugal or Spain? The answer is NO. Currently namban named for cuisine means green onion/scallion. When Portuguese and Spanish missionaries brought new food, new cuisine to Japan, these foods or cuisines used many onions and chili peppers which Japanese did not use for their cuisines. Unfortunately onion was hard to get in Japan till 19th century therefore Japanese used the green onions/scallions as the substitute.
Anyway, duck & green onion is a very good combination. There is a Japanese idiom; A duck came with a green onion on his back. Duck is considered as an easy prey and green onion is an extra bonus.