The notches in the front sight are for a hood. Most of the Rigbys which I have looked at are missing the hood too so don't fret too much about that.
Is that front blade made of bone? Makes for a nice night sight I would think.
I think the Rigby restored by Dorleac has one so you can see what they look like. They are rather long and unattractive so I bet most of them were removed and subsequently lost.
Do you know any of the history of the rifle prior to your uncle getting it?