Even the very low salt Kallo organic stock cubes contain sugar - but only 0.2g per 100ml i (total carbs 0.9g per 100ml) in the case of the chicken ones. Oxo chicken stock cubes, by contrast, have dried glucose syrup as the second ingredient and sugar as well. but reckon on 1.5g carbs per 100ml.
Granted, the mainstream stock cubes often contain vitaminized flour, but they may contain things like monosodium glutamate, potato starch, iodised salt, disodium 5-ribonucleotides, maltodextrin, dextrose, caramel syrup, hydrogenated palm oil, silicon dioxide, autolysed yeast extract, ammonia caramel, disodium guanylate, maize starch, E621, E631, E627, artificial food colour (E150), smoke flavouring, and smoke..er, how ?
Stock pots.. glucose syrup, maltodextrin, caramel syrup, yeast extract, potassium chloride, sugar, sunflower oil, potassium sorbate - generally, they seem to have fewer additives but they aren't necessarily less carby.