It could also be low due to heavy use. I seem to recall one conversation where somebody said if you have a choice in setup between ever so slightly high and overshooting to be a tad bit low its better to leave it alone and let it wear in over time. I think a mechanism that can be adjusted over time makes more sense, but that's what they said. I'm not sure how being a little bit bad in one direction would be better than being a little bit bad in another direction, but that was the gist of it. It reminded me of shooting pool with people who ask if its better to miss by a little bit or miss by a lot. "Um, you missed. That's all." LOL. " Its still my shot."
Like every other surface on your lathe it will wear over time. As to the clamping plate on the bottom. It may be original. I have three lathes with similar tail stocks and plates for clamping in place. There is nothing special about any of them. They are all just the right size piece of pretty generic plate with a hole tapped through it.