Mine were etched on with a Dremel using a carbide bit with a tiny ball on the end. They also filled out a log book with the date, type and size of cylinders along with my name and drivers license number.
They were recertified Nov 2012 at a full service propane and fuel supplier that does rural delivery. They also sell gasoline, diesel, E80, Ethanol free gasoline and fill propane cylinders on site. They charged $5 per cylinder. I had 2 20 lb cylinders and one 6 lb cylinder. The 6 lb cylinder was several years out of date but I was always able to get it refilled when getting other in date cylinders filled since I always had it filled last and they never checked the date since the others were in date. Cost me $15 dollars plus refills on the cylinders.
This past summer they burned off the propane from 2 30 lb cylinders that had been improperly filled without being purged by 2 other retail propane sellers. They removed the valves, put a type of alcohol in to absorb moisture. Emptied the alcohol, reinstalled the valves, properly purged and filled them for under $50.