Jeff, Ive been going through all I can find on the ada, and after hours cannot say I have a definitive answer, but thought I would share some notes and references that may offer some ideas.
From "Africas Ogun:Old World and New", Sandra T. Barnes, 1997, p.60.
According to Smith, "...swords were of two main types: the heavier single bladed and eccentrically curved engbe, and the ida, usually double bladed and either with an elongated leaf shaped blade or approximating to European or near Eastern types. Other varieties of swords and knives were also used, such as the short Jomo, the tanmogayi (sabre), ADA, ogbo or ele (cutlass) and obe (dagger). With the exception of the obe, all these were designed primarily for cutting rather than for thrusting or stabbing."
The ceremonial sword was the ida type.
from R.S.Smith, "Kingdoms of the Yoruba" (London, 1976, p.145)
see also, "Yoruba Warfare and Weapons" also R.S.Smith ,1973,
'Sources of Yoruba History', ed. S.O.Biobaku, Oxford,
Clarendon, pp.224-249
Other notes from other sources:
EBEN: The Benins ceremonial sword , shape of sword is like a fan or an oar, iron blade, large ring shaped brass handle, edges of blade are blunt, kept in altar for royal ancestors.
ADA: ceremonial sword, Benin kingdom 18th century, had ivory handle, interlaced motif, iron blade brass inlay ENGRAVED FIGURE OF A BIRD over brass inlay, special mark on blade to represent magical power in iron.
It is noted that the ADA is carried by a bearer in front of the king (Oba)
The markings are essentially story emblems significant to symbolism for regalia. There are a number of symbols significant, one is the mudfish (actually a catfish, clarias anguillaris) which seems to figure into the royal mythology. While there are no fish symbols on the blade of this, there are the fin or flipper type scaled images....and these seemed worthy of note.
The bird has meaning representing the mystical powers of women, whose support is needed for the oba to rule effectively (why is this not at all surprising
) Apparantly there is a great deal of emphasis on the Queen Mother.
Apparantly brasswork was extremely important in much of the regalia, and I found references that note that ada's and brass masks were introduced to vassal lords as emblems of authority.
I think here might be a clue.....while this example is entirely in brass, or bronze, perhaps it was intended diplomatically in this sense, carrying certain symbolism representing royal imagery, yet of lesser degree than the ceremonial examples for the King and his immediate court.
I think more study on the art of the Yoruba; Benin and Nigeria might help in finding better understanding of the symbolism.
All the best,
Jim