I don't have an extra /6 FW laying around to check but on a typical 10.5 V8 factory FW there is a step where the working surface meets the body of the casting. As a FW is used and reused, that distinct ridge disappears with resurfacing. I take that to be like wear indicators on tires. (Bars or holes) I have heard that is a good way to judge if a FW is serviceable or not.
Always better to surface grind FWs, not lathe turning.
