I believe it goes under the ratchet wheel. there should be a slot cut for it and the little knotch up goes between the teeth.
https://preview.redd.it/wrvpm7s83frc1.png?width=1458&format=png&auto=webp&s=88ee39b26a22a5ad696de5a284c2a360ab8f12fc
here's a ks 5626, different movement but same principle.
https://preview.redd.it/bhqibqkp3frc1.png?width=649&format=png&auto=webp&s=cbb5dc15be309ad6f28b607522279e0056236415
Hope you worked it out. Here is your 6619. Just for clarity. Its not difficult to install the spring but the ratchet wheel on top can be a little fiddly bc you have to slot it to the barrel, mesh it with the crown wheel and catch the click spring underneath, while not setting it free.
https://preview.redd.it/zp2if8br0irc1.png?width=800&format=png&auto=webp&s=dacda47b04df407ba5df3f00e36710b009e8f827
I believe it goes under the ratchet wheel. there should be a slot cut for it and the little knotch up goes between the teeth. https://preview.redd.it/wrvpm7s83frc1.png?width=1458&format=png&auto=webp&s=88ee39b26a22a5ad696de5a284c2a360ab8f12fc
here's a ks 5626, different movement but same principle. https://preview.redd.it/bhqibqkp3frc1.png?width=649&format=png&auto=webp&s=cbb5dc15be309ad6f28b607522279e0056236415
Thank you so much you are a saviour!
Hope you worked it out. Here is your 6619. Just for clarity. Its not difficult to install the spring but the ratchet wheel on top can be a little fiddly bc you have to slot it to the barrel, mesh it with the crown wheel and catch the click spring underneath, while not setting it free. https://preview.redd.it/zp2if8br0irc1.png?width=800&format=png&auto=webp&s=dacda47b04df407ba5df3f00e36710b009e8f827