Rentals are generally the 200 to 300 CFT sizes due to government mandates. Renting a smaller tank just does not make good economic sense. I asked the manager at the local welding supply store about that and he told me that they usually never check the smaller tanks for ownership especially if you are a regular customer. As long as it does not say RENTAL on it, you are good to go!
The tanks I bought were owned by a company that is now out of business in the Detroit area and so the LWS had no trouble exchanging them. The guy who sold them sells tons of them and buys from auctions. I figured I saved around $700 on my tanks by buying from Craigslist off this guy. All are 125 CFT tanks and when bought were full. He carries argon, 75/25, and O/A tanks.
All the tanks I have had raised lettering and that has never been an issue with getting refilled.(actually exchanged) The only thing that gas companies will not do is exchange tanks from another vendor....like Airgas tanks at a Matheson dealer. Here is a cool video from a weldor friend Freddytk421......http://www.youtube.com/watch?v=ndGB-Ww47M4 that explains the legal issues of tanks.
I do agree that you should do your homework though. I never would buy an adulterated tank. Also make sure that the tank has been tested within the last few years. Some folks have tanks that have been around a while and sell them. Then you get them and find out that they need to be tested. You have to pay for that test.
Here is a link to a US military site for the markings on pressure vessels.....http://www.usamma.army.mil/assets/do...20Markings.pdf