It seems you were referring to NH3 cylinders. I believe the only NH3 you will find in that size if any would be used as a refrigerant. The cylinder itself would be silver, with a DOT sticker that would say R-717. There should also be a large diamond shaped non-flammable gas sticker on it also.
This link has a picture of what the label should look like, of course yours would say R-717 not R-12 http://www.thehvacsource.com/Articles/labeling.cfm (http://www.thehvacsource.com/Articles/labeling.cfm)
here is a few different types of NH3 storage tanks
http://hansontank.us/ammoniastoragetanks.html (http://hansontank.us/ammoniastoragetanks.html)