Can't provide the link via the rules, but here is the quote:
If you tried to find horn tips for your English Longbow you would most likely find Water Buffalo horn tips. Not at all authentic. The single surviving horn tip from the Mary Rose appears to be deer or stag horn. If you wanted to have authentic horn tips for your Longbow you would use deer elk or cow horn. Here they are and pre drilled. Set of 2 pre drilled longbow tips.
Just curious, as I really don't know either way, but I thought they were supposed to of been all horn, presumably cow horn?