Don't worry, we all get our turn.
It doesn't help that "HP tanks" include a small number of no longer made tanks that are different than the rest, and an awful lot of sellers have no knowledge of what they are sell.
Below is a link to an XS scuba page that shows how to read the cylinder markings of various tanks they sell. Although some of the markings you will only find on new tanks most have been used since they were stamped ICC before it became DOT. Make sure the service pressure is below 3500# and you avoid repeating this issue. Know, or have a cheat sheet handy might avoid others.
Cylinder Markings — XS Scuba - Everything For The Perfect Dive