Terminology is odd as my SA called the WG oyinx set he priced for me a MTO. He said it is pre-approved and always available, just at a premium. So strange SAs call them differently.I think MTO is a piece they have had in regular inventory but now has to be ordered and it will be at the regular price like the Cosmos onyx you mentioned. SO is anything that you have to pay the 30% (or so) premium to have made. The list of Alhambra pre-approved is still SO as it commands the extra premium. That's how my SA explained it.
I do think it's absurd to charge a SO premium to stick onyx in wg when they already make Alhambra settings in wg and they cut the onyx clovers!
Regardless of what SAs call it, he said you can get the "pre-approved" pieces such as WG oynx anytime. My SA just pulled up pricing for the WG oynx pieces from the computer and wrote down the prices for each piece one by one. He did not have to get any special approval or a quote; he just showed me it was all there on the computer.
The "SO" pieces (his terminology) such as WG blue agate he said he had to first ask and get approval/pricing. That took a few weeks as I recall, he did not just bring up the pricing on the computer. .