It's a 2018 S3 and, yes, it's the double diagonal bars. One bolt just about came out with around 0.5mm to spare but the other side was about 0.5mm longer and had to be cut off at the head. Clearly the diagonal bars are not symmetrical. Luckily the replacement bolt fits perfectly with loads of thread inside the boss, only about 3mm less than the original one. Caterham are out of stock of the 7/16 1-3/4 UNF bolts.