Both of my Impact X models are the four black bands. No white markings. I simply believed it to be a change in the manufacturing process as the gun evolved. The marks are just relative index marks to allow controlled setting/adjustment of the VSA knob. By going to the black ring index markings they are able to place these index marks on the part during the machining of that part, and eliminate a process step of applying a white marking compound. It makes good manufacturing sense to me. Plus, white marking compound applied on an anodized surface can probably be worn/rubbed off in spots eventually if it has much abrasion applied over time and handling.
Just my thoughts/assumptions.