There is a stout V spring which fits in a space in the frame in front of the trigger. The place for the trigger return spring is often an open spot in the frame visible when the cylinder is removed. The old spring might still be there or maybe it fell out. Broken or missing trigger return springs are one of the most common problems with pin fire pocket revolvers. Fitting a new trigger spring often involves use of a vocabularlly unsuitable for women and small children.
On some of these revolvers, the trigger return spring is found after removing the trigger guard.