My riverside picnic in Victor Constant country

What could he eat on duty, riding the rough roads of the Champagne province ...?

You might be surprised to hear that the sandwich was not unknown in France at the time and it was very much like the ham-and-cheese baguettes you'll find by the Seine today. Here's a description from Murder at Cirey.

He asked the farmer’s wife for bread and cheese for breakfast, packed so he could eat it in the saddle. In addition she had fried him up some chon, little pieces of pork crackling that were typical of the region. She had sensed his urgency to be gone but she prepared them with her usual attention, then split the narrow loaf and inserted them with soft pieces of Chaource cheese.

‘Am I to tell Cavalier Renard where you’re off to in the dark?’

‘No, madame, for I don’t know myself.’

‘Another dash around the country, is it? Well, make sure you don’t get yourself confined to barracks this time, or we’ll never see you here again!’

‘You’d miss me?’ he said with a grin.

She wrapped the long sandwich in a cloth, a half-smile on her face. She was not yet twenty, a good-looking woman well into her first pregnancy but with the bloom of youth still on her smooth cheeks. ‘I’d rather billet you than many another — you like my food, you don’t roll in too late at night and you leave first thing in the morning. If we saw enough of you we might get to miss you. Just make sure we do.’

He laughed. ‘I’ll try.’