I believe Leon is correct that SGC will grade all whites without backing as "A".
The colored silks, which you know can include white or whitish-looking examples, have no backing, have the factory and manufacturer info as Leon mentioned, and are longer in size. IMO, they are not worth grading, and SGC is all over the board on these. This is one set where I routinely crack them out of the holders, as the silks are durable and not easily damaged and again, the grading is inconsistent, IMO.
Here's one of mine:
Edited to add: To be clear, I didn't mean to suggest they are not very valuable and thus not worth the grading--I love 'em and they will fetch nice $$$ when they look nice. Rather, I don't see the value added in paying for the privilege and I think you will end up disappointed and/or baffled by the grading results.