I'm trying to find out the part I need. The latch handle broke

Thanks for posting. Our 82-107 should be able to replace your old thumb latch. However, note that a damaged thumb latch can sometimes be indicative of a broken mortise lock. To see if that's the case here, remove the handle set from the door and try manually 'throwing' the lock with a flathead screwdriver. If it doesn't budge, then the mortise lock also should be replaced. See if our 82-265 matches up.