Possibly a yuzu - thin skin, looks like an orange but more grapefruity in flavor.http://en.wikipedia.org/wiki/YuzuHowever the juice and zest is excellent as a flavoring - especially in sauces, marinades, etc.
My neighbour has a tree with some sort of fruit that looks like a small, thin skinned orange but doesn't taste like an orange. A bit tarty - I reckon I could get some of those and use them in this recipe.
BTW - meant to say above - achookwoman's recipe says to add 3/4 of the sugar to carrot/lemon mix. I have needed to do similar things before and so worked out that my TMX bowl (without lid) is 1.080kg. So, if a recipe - usually jam/marmalade - says weigh the resulting pulp and add an equal/three-quarters of the amount of sugar - then the calaculation is easy !!!!Should put it in tips while I think of it.