[Red Steel]

They probably, out of exuberance, printed all govt docs with “Republic” as soon as they declared Independence. They sure didn't want to continue with anything that had Britain associated with it. I was just looking at the time-line.

Yup, that's the most likely scenario.

[PowerPro]

We should find paperwork from that year to see when they started putting that on their official documents.

What did they call themselves for the entire year after the break with Great Britain?

[Scanian]

What were they going to call themselves in the meantime? “Republic” was the obvious apellation, as all the newly independent African nations called themselves republics.