[Jim McDougall]

Most interesting notes on these cross like fixtures on the Omani terrs, and as has been noted, these issued in pairs on the shield are clearly intended not only as decoration but to secure the holding strap.

Earlier there was a note questioning if the cross would be something which would appear on a Muslim item. Actually, the 'cross' , while of course having a well known presence in Christianity, in this case is probably much as the forms of cross known among Tuareg tribes which are not intended as a religious symbol but directed to the four cardinal directions of the compass.
These Tuareg crosses, most commonly known as 'the cross of Agadez' (in Niger), were prolifically produced (at least 21 types with certain tribal significance) since the 16th c.
They were of course heavily traded and certainly would have been present in routes trans Sahara and into Ethiopia, then of course networked into Zanzibar.

With the Sunni Muslims among Tuareg tribes, these are often regarded as talismanic, the four arms to protect from evil 'from the four corners if the world' and the center representing God, the center of all.

As these types of cross were of this character and entered the trade systems into Zanzibar, it seems plausible that similarly shaped examples might have been added on these shields in that thought.

Another thought suggested for these, many of which are with the circle above, is that the Egyptian ankh may have had influence.