The way it was explained to me, in a article in Airgun World I think was.
Each piece that had to be unique to the gun, body, screw in spring guide, trigger guard and barrel catch, were put together pre bluing.
Then machined to fit, then stamped with a batch number, taken apart and send for bluing.
When they got back the batch number was used by the assemblers to identify which parts went together.
This is why buying these bits are a lottery on how well they fit.