They weren't sandblasted.
The rough finish in the slots is just rough casting.
Once the wheels were cast, Toyota would machine the face, smoothing it out, but leaving the rough surface in the slots.
From there, they would add one of 3 finishes for the US market.
1: Silver with silver cap.
2: White with white cap.
3. Slots masked and painted silver, while leaving the face machined and adding clearcoat. rare, I've only owned one set. they came on an unmodified 87 turbo, with the option listed on the window sticker.
It doesn't show up listed in the sales brochure however, but some of the pics show them.
Chrome or polished sawblades were sent out by the dealers to be stripped and either polished or plated, this still happens today. It's just something a dealer does to be able to mark up the price due to the option price being much more than it actually costs to plate the wheel.