Your brilliant Conway! The kana characters have an origin in kanji characters. For example, Japanese submarines used the iroha order for numbering. In formal documents, 伊 is used instead of イ. However, the conning tower would use an イ followed by a number.
伊 = い, イ = I.
呂 = ろ, ロ = RO.
波 = は, ハ = HA.
仁 = に, ニ = NI.
For the rest, click on the characters at the link below. In the column to the right, find the characters 字源 [character source] and the source character will be shown.
いろは順
Now, back to swords. You answered something that has been nagging me for awhile. I remember seeing a black painted 呂 prefix on a sword tang and was wondering why they would use that character. I thought maybe it was the first character in the name of the shop or the fitter. I now think one of the swordshops could have used the kanji source characters as a prefix. What do you think Conway?
cc @SteveM