I remember that little kit that came with the Blackburns, but I don't remember any Blackburn racks that didn't have the little hook built into the bottom brace an inch or so above the hole for the dropout. The pannier should be able to use that without any trouble. I've done that with my original ones as well as the modern ones.
You haven't been too specific about the Frejus dropouts. If Campy, the longer Campy hex bolts will work with the dropout screws, and virtually any of the bike specific allan bolts too. I believe water bottle boss bolts are also the same thread. Most bike shops should have those threaded bolts in their inventory. If that works, you can thread the bolt into the dropout. If the fender rack boss isn't threaded, you could either find a smaller bolt that fits or have the dropout tapped for whatever standard bolt you'd like (Sacrilege?).
For my threaded Campy dropouts, real touring and self support, I made sure the bolt was long enough on the side away from the freewheel that I could thread a lock nut to the inside of the dropout for insurance. Because of freewheel tolerances, I had to cut the bolt flush with the inside of the dropout on that side. Use washers, carry spares just in case and if touring, your son should be carrying at least a basic tool set, either allan keys or a Campy T wrench (the correct vintage accessory!), and check the bolts for snugness regularly. Chainring bolts too if riding rough roads!
Dan Artley in Parkton, Maryland USA
... Here is my problem. The Blackburn (rack) is fastened to the rear dropout with allen bolts, and they are missing. Also missing are the little hooks that went on these bolts to clip the panniers.
Would anyone have these that they would be willing to sell? Would anyone know of a shop that would have these for sale? I have started checking online without any success.
Thanks for your time.