The ones I have seen, require a milling machine, and a tee slot cutter.
The bolt shroud is jigged up, the slot is cut.
Steve Acker did an article on the process in Home Shop Machinist magazine a short while back (within the last couple years) and you can very likely find an index online that will show the actual date of the issue.
I suppose a decent hand with files could do the job, but it would be easier with a mill.
IIRC, Brownells sells the parts and the jig.
(checked their site. You got the Chapman Safety? Description says it includes complete instructions. If you did not get them, maybe shoot an email off to Chapman and see if they will send them)
I probably have the issue around in my piles of magazines, but it would take some digging to find.
Cheers
Trev