The tyre is 700 x 25C. Two inner tube sizes were available: "700 x 20C to 700 x 25C", and "700 x 25C to 700 x 32C". Oddly there is an overlap, and so those with 700 x 25C get to choose. I chose the former to reduce the chance of having a kink.
Yet I managed to get a kink anyway, or at least it looks like it. I did ride just fine for 1500 km or so, but a hole eventually developed exactly at the edge of the odd-shaped bulb in the image.
How does one end up with this inner tube kink, and what precautions should one do to avoid it when fitting the tyre? After inserting the first bead, I inflated the inner tube slightly, inserted the stem, pushed the rest working from both directions while ensuring it's not twisted, inserted the second bead of the tyre with levers, applied soapy water in a couple of locations to get a good seating, then inflated to 90-100psi. This recipe failed me in some step. Do you see it?