On a 1970 AS that I had there was a lock that screwed onto the threaded pipe, down and over the bolt, and when locked made removing the tanks very difficult. (See the picture sent by OB Bambi above for the bolt I mean) I am sure it was a after market item. Made of brass, a few ounces in weight, maybe 1 1/2 tall. When I replaced that AS with a newer model it would not fit under the propane cover. Was a good idea though. I have never had bottles stolen but a few dollars spent on protection is not a bad idea.