I have a Century Arms L1A1 Frankengun, almost identical to the one under consideration in this recent thread:

http://www.ar15.com/forums/t_6_7/465581_Looking_at_a_FAL_L1A1__UPDATE_WITH_MORE_DETAILED_PIC_S.html

Its a British L1A1 kit on an Imbel receiver by CAI. Its at a gunsmith about 2 hours away, that is familiar with FAL type and com-block type rifles, and he is doing some repairs and updates to the weapon. I want to add a flash hider to this neutered barrel, no slot or threads left. Gunsmith says he can thread the barrel either right hand or left hand, but does not want to install the original L1A1 flash hider, which would require pinning to the barrel or milling a new washer slot in the barrel. He recommends that I buy another of the threaded repro Browning flash hiders from DSA and let him install that.

That's exactly what we did recently to a sister rifle whose Imbel receiver was cut for inch mags, for a measure of authenticity, essentially to keep, "the look". However on this rifle the inch pattern Imbel mag was cut for metric mags only, so I don't care about the original "look" so much. SO, what other .30 caliber flash suppressors or possible hybrid muzzle brake would look good and function well on this rifle?  I am considering the short Steyer FS or the BE Meyers 249 type. I really don't want a muzzle break that going to blast most of the noise sideways at other shooters. I would consider other devices intended for AR10 type rifles.

Your suggestions please.

One alternative might be to use a 2nd nut as a jam nut for the muzzle device?

It's what is used on my K2 and hasn't loosened in 2 decades.

Imbel or Belgian would suit your needs.

threaded repro Browning flash hider from DSA for looks. Short Steyr FH for actual SHTF usefullness (wire cutter ability)

"Imbel or Belgian would suit your needs"

Are these both threaded without the washer slot? Any sources?

Yes, both work with just threads- left hand thread to counteract the rifling torque- and no need for a washer.

Check with Arizona Response Systems. I know Mark has them in stock, and he can easily install one.

Thanks with all the suggestions. Even though the look is not correct, I think I am going with a short Steyr flash hider.

Well, it is a Frankengun.