This is a little late but I have info. I own a Cobray M-11. The older Semi's fire from an open-B. The newer fire closed. Both are extremely easy to convert. Expect to pay over a grand for selective fire, you will get a wire stock too, ooooooh.....
Look around, you can find a preban Cobray M-11 w/threaded muzz for around $3-400.00.
Any ????'s feel free to get back n' ask. I have plenty of info. and if I don't have an answer, I have plenty of sensitive reading material to find it.