Tony,
Sorry for being so long in replying. I got side tracked twice when I was about to answer. If you have only the 671A cross slide, my feeling is that it would be worth something to someone who had the other parts (the front turret, rear tool holder and two carriage stops) that make up the complete 670. For price reference, last year I bought (complete) the 12" version that only has a Craftsman model number (because Atlas revised it about the same time the 1/2" bed lathes came out so they never sold the early version). I paid $300 to a seller from whom I had already bought several other accessories. When he originally put it on eBay, he was asking $400. Of that, if I were to break out the parts individually, I would price the cross slide at about $150 out of the total. However, I do not know whether the 10" and 12" bare cross slides are the same Atlas part or not because unlike most of the heftier Atlas castings, the part number does not appear on the machined casting. And I have never been able to locate the Craftsman parts manual for it.
I'm not sure that it would be of practical value on a 10" without the other parts as I haven't checked the height dimensions. But it's possible that the tool height requirement might be within the range of an AXA class quick change and the older 4-way turret for the cut off tool on the back position. On a 12", you would need a ball park 1" spacer in order to use it.
Robert D.