I have no idea how common or rare these are, but I've never seen one. I could have used them years ago for the A7's I had for my P.A. system. Cool that they lock too (not that they're a high security lock). You know what they say about the effectiveness of most locks--locks keep out honest people.

http://www.ebay.com/itm/NOS-Qty-5-AL...16.m2518.l4276