You don't need a whole new trigger unit, friend; you just need whatever isn't clicking into place properly, or perhaps the spring (there will be one, never fear) that makes it go into place....
Take the trigger mech out and analyse what it's doing or not doing and you'll be able to see what's not up to snuff.
It ain't rocket science, it's a rifle which is ready for the Old Age Pension. YOU aresmart enough to figure it out. If you can't, get back on here and someone will help. It's just a standard double hammer-hook SA mech.
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