The Red Arrow
All I can find is
this - (which is what I'd already come up with myself on searching through UT - there's no mention in HoMe)
I think, though, that the account in Cirion & Eorl was written after LotR, so the question that springs to mind is, was this story already in Tolkien's mind when he wrote of the Red Arrow in LotR, or was it based on something else, & the later account 'invented' as an explanation
within the secondary world?
It is certainly an 'odd' symbol to use - how old was the Red Arrow? There seems to be a history behind it which Theoden was aware of. He only had to see it in Hirgon's hand to understand exactly what it meant. It seems from Theoden's words that it is a sign of absolute desperation on Gondor's part - 'Has it indeed come to
that?' It seems that it declares Gondor to be in extremis, at the absolute point of complete destruction. Certainly, Hirgon could have told Theoden that Gondor was in deep trouble, but it seems that Denethor felt that would not be enough & he had to send the Red Arrow as well. It does seem that it had a very powerful symbolic value, & to be calling on the Rohirrim to fullfil an
oath.
This is interesting in the light of the events of this & the previous chapter - oaths run through both, ones held & ones forsworn.
In passing, I also wonder what the 'mark' painted on the arrow was.