The 77 should be fitted with the 58 Duro. This might be quite a common nib to try and find, you can keep an eye here:
http://www.conwaystewartpens.co.uk/Nibs.aspShame its lost its original nib, it was a good one! However, if its a pen bought to use then the warranted nib might be equally good (the Duros can be wonderfully smooth but are stiff), depending on your taste. I have a couple of pens with 'Warranted' nibs and they write beautifully.
I can understand if you find this less satisfying than using an original nib, especially as it was a Duro.
Greg
Greg