There is nothing to believe, I already said in the thread 16mm in the middle, I am not going to nit pick over a .12mm smaller.
Been to Walbro many times. The 603 was made for 30 to 50cc engines the HDA223 was made for 50 t0 100cc engines. I have the 50cc model of C1, ( 46cc ), it has a copy 19mm Dell PHBG w/ 1.01mm main jet. It's alot of carb, hard to tune it, and I hate changing jets, messing around with atomisers, its a waste of time.
The Walbro runs up side down, no jets, floats, float bowls, needles, gas turn off valve and it don't leak all over the ground. I have a little screw driver in my pocket, 1/4 turn I'm jetted and ready to Rock and Roll.