Correct me if I am wrong. I think I have found a way to track sales of both the models on sale.

Depending on how many items you ask for in order screen, it lets you go ahead or displays "stock unavailable".

For the Mettalic Silver, It seems 57 items are available at the time of writing. and for Black Anodized, it is 108. I have to take a reasonable assumption now to get the sales numbers (may be 200 at beginning?).