I have an old M4-1000, and as some of you probably know, the attachment method it used wasn't the best. It used a spring that just sat in the valley on there 18T mount and was just held in place with friction. Well over time the metal on the pring just grinds away and will get flat and stop holding. So it was time to send it in to get replaced, so I called customer service and asked the guy if it was possible to change the mounting system over to a new mounting system like 51T. Of course the guy didn't know but he wrote it up as just a latch repair with a note to replace it over to 51T if possible. Well fast forward a few weeks and I've heard nothing at all and I get an email saying that they were shipping it back. So I'm not expecting much, just the spring replacement. It shows up and it has been refinished, the old mounting hardware cut off and the 51T mounting hardware welded on. I need to buy a 51T flash hider to try it out, but I'm super happy with the result!