To the best of my know;lege there were No BFAs made for the SKS. Some have claimed that the cap to the butt trap cleaning kit was a BFA, but it is only a cleaning rod guide and will whoot off if used as a BFA.
There used to be a company that made BFAs for the SKS that clipped around the front sight like the clg kit cap only it was a heavy duty steel thing painted orange. they quit making them because there were to many problems with them.
Most of the Viet Nam reencators simply thread the muzzle and screw a plug in to the bore. If you do this the proper hole size is drilled with a #13 bit. works with both the SKS & AK when using chinese or east German blanks.