@Old Man Riva
It's a pretty easy process, I emailed them directly and one of their sales guys (Brian) came back to me pretty quickly. I've had both a US Custom 44-94, and a US Classic Series 44-64.
As you say, essentially the Classic series came about as US series basses that they could make in batches by reducing any custom options. Classic series also has a reduced model choice too. As far as i'm aware, they typically only offer the Classic series in Sunburst, however sometimes they'll do special deals on custom colours too, which I think they're doing at the moment.
The US Custom series will allow you full custom options, all sorts of things like tuners, pickup placement, chambered bodies, fancy woods etc etc
Remember that postage is about $120-150 extra on the price, plus your VAT cost when it's imported.
My basses were both excellent, I have my 44-94 still, it's fantastic! I sold the 44-64, it was excellent, and sounded lovely, but while it is largely a 60's spec, it is a slightly more modern take, so put the money towards a Moollon. That said, I will get another 44-64 at some point!
Anything else, just send me a PM
Si