The join is inside the port and once the finned ring is in place it's not noticeable. Without the extension there was no space for the ring.
Another set of siamese I inherited (with a sadly neglected '59 SR) was made up in a similar way, before finishing of course. The tubing used for the main bends is smaller dia. than the welded end pieces, which match the port id and the welds are obvious when fitted. I have no idea who made that set but guess the very tight bend on the lhs short piece proved too difficult.
Terry