I'm sure they will have 34 separate SKUs for each of the combinations on their website. That said, they will likely sell replacement straps at some point since leather wears out over time and stainless steel can corrode or break.
Honest, I think this would be far easier for Apple, and far better for customers.
Think of it like iPads or iPhone and cases.
Apple can just make the different watch bodies, in BULK of course, and in their own nice little box, they can pack them up small, and neat in a lovely box, without any worry about what combination will sell well.
Just make and show on the website which of the bodies you want.
In the same way now, you pick the iPhone size, and it's colour and it's memory size.
Then they just make the straps, separate items, like iPad/iPhone cases.
Apple don't now have to bundle a red case with an iPad, a Blue case with an iPad, a Green case with an iPad etc, that would be a pain, so why complicate things and do it with a watch?
It would be far far easier from Apple from a stock planning and distribution standpoint to simply let you pick which body you want, then scroll down and pick which strap you want, and simply ship you the two separate items, and you just lick in the strap when you open the pack.
Apple never had to worry about making too many of combinations people don't like.
And people don't have to buy a strap they don't like, only to then have to replace it with the strap they do like.