I see no misleading here, to be fair. Item description clearly says 509 (STD) board. Photos clearly show 509 board. Seller took the effort to actually take decent pics for each sides, not all do this.
There is no promise anywhere that you get 519 or 529 board. Seller may not know much about 3458A, it doesn't mean listing have a problem though.
Age is a good thing in this particular case, as there is a chance that this reference was aged and stabilized over all those years. If I'd have a choice to buy LTZ ref from 1988 or new one from 2016 for same price and parts otherwise, I'd go for 1988 one. There is high chance that such reference can be more stable than brand new 519 A9 Ref
Price is okayish too, you wouldn't get LTZ ref much cheaper even if you do it all DIY.
If you think that getting A9-HFL 2ppm/year board directly from Keysight for 2K$ is much better, and does not need rigorous and time consuming testing with verified equipment - well, you are in for surprise