That's a sweet deal for those tanks.
There should be an inspection date stamped into the metal up near the top of the bottle. If you can't decipher all the crap, take it to your local oxygen/welding shop where you get them filled, and they'll be able to tell you.
As for what it means if it's expired, well, that all depends on what the refiller's policy is. I got an expired tank as part of a craigslist haul once. My particular welding place actually refills your own tank and hands it back to you (some places do exchanges), so they could not legally fill this one, and they didn't do onsite testing there (some places do), so they had to send it to Richmond, which took about 3 or 4 weeks and cost ~$20.
So, just ask your local place what their policy is. They might just take that one off your hands and hand you an inspected, filled one...or they might say that you need it inspected...or they might inspect it for you...just ask.