I have a 600AT New Haven that the safety broke on (big surprise). My research found that the safety switch and the block inside of the gun should be the same as for the 500. The problem that I am having is that when the block is installed, you can never release the hammer. The trigger is always blocked from movement. The switch and slider plate fit the cutout in the receiver just fine, and the switch slides back and forth into the detent positions just fine. I am stumped. I have tried installing with the hammer cocked and the hammer released. And yes, I tried installing the block in both directions - the proper direction is the only one that will allow you to slide the safety back and forth. My understanding is that the 600 AT is a low cost version of the 500 (I bought it in the late 1970s) and I find it hard to believe that Mossberg would have a "special" part for the 600. Here is what I bought - I have to think that it is something that I am doing wrong... https://www.amazon.com/gp/product/B004K3TMCU/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1 https://www.amazon.com/gp/product/B00HS5V9UA/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1 Thanks in Advance!