There is no info in the S&W catalog that indicates any major changes between the two pistols. I would imagine the -1 magazine has a cut out for a magazine disconnect that will cause a possible malfunction in a non dash 4516. That would be my best guess.
Next time you're at the range load the pistol and drop the magazine. Try to shoot it. If it fires there is no mag disconnect. If it doesn't it has one.
Hopefully somebody here has a 4516 no dash model and can tell the reason for the warning.