I finally received a Giles Berthoud handlebar bag for the Randonneur. It's the Model 25 in gray, which is their medium size and features elastic ties for the pockets instead of leather straps.
Ostrich handlebar bag on my own bike is more drab and also more bulky.
one of these if going without proved impossible, but I have since been warned that using it with the Nitto lugged stem (as opposed to the regular Technomic stem) is not a good idea, for complicated reasons related to clamp compatibility that I won't go into here. An alternative would be Berthoud's own decaleur, but it too apparently presents issues with the lugged stem - plus it is fairly obtrusive without the bag on. A headset-mounted decaleur is not possible here, because it would sit too low. So I am seeking a solution, and also wondering whether securing the bag at the bottom would in itself solve the problem. The bag is not nearly as large as my Ostrich, and I know that some do manage to use it without a decaleur. Sharing of experiences in this regard would be most appreciated.