The Sandwich de Comptoir is defined by where it is eaten, which is standing up at the zinc bar with a coffee or a small glass of wine and no plate in sight. The comptoir is the counter of a Paris café, and the sandwich named for it is whatever the café keeps within arm's reach of the espresso machine: a half-baguette already split and filled, jambon and butter most often, sometimes a wedge of cheese, sometimes saucisson, wrapped in a square of paper and handed across in the time it takes to pull a café. What makes it its own thing is not the filling but the posture. This is a sandwich engineered for the upright minute between other things.
The constraints follow from the bar. There is no table, so the sandwich has to be eaten one-handed while the other hand holds the cup; that rules out anything that spills, slides, or needs both hands to manage. There is no plate, so the paper is the plate, and the build stays tight enough not to shed its filling onto the customer's shoes. There is no waiting, so the sandwich is made ahead and stacked under the counter glass rather than assembled to order, which sets the upper bound on how good it can be: only as fresh as the morning's delivery, only as generous as a café that sells volume can afford. The half-baguette is the right size because it is finished before the coffee goes cold. Butter does real work here, sealing the crumb so a sandwich that has sat an hour under glass has not gone to cardboard by the time it is sold.
The variations are the café's small menu rather than a tradition: jambon-beurre, fromage, saucisson-beurre, an œuf dur with a twist of salt for the customer who skipped breakfast, occasionally a mixte of ham and cheese together. Each is chosen for the same reason, that it survives the morning under glass and eats clean while standing. The Sandwich de Comptoir belongs with the place-named sandwiches the catalog groups under Regional Specialty Sandwiches. Its specific contribution is the counter itself as a design brief: a sandwich shaped by being eaten on your feet, fast, with one hand, and never sitting down.