That would be a good question for VTL.
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When I sent requests to VTL about their equipment, not only did they respond quickly and in detail, but also asked me to call them and discuss things further with them. So no problems at all in my experience, and just as good a response as any high-end manufacturer that I've contacted (though perhaps during the holidays, like most businesses, they may be a bit slower than usual).
As to the main question...while I'm not an expert by any means, there are certainly changes made to the design over the years (as an example, there's a ground near the tubes for easier biasing in newer ones), so the older ones will not be the same as the newer ones. Whether the difference involves sonic changes that are easily audible is a separate issue...I have no idea as I've only auditioned them recently, so have no experience with older ones.