Early on they held the bolt on with their thumb. One hand would be holding the back with the trigger, the other would be holding mid-stock & they’d reach their thumb over & press the blot down to hold it in place. There’s a tapestry that shows the method; it’s a scene from a legend, but other then they guy is shooting a rose, it shows the technique pretty well.
Later on they started using little horn or metal tabs that were secured just behind the release & pressed down on the bolt to keep it in place. See item 15 on this page