Given parts book book image below it looks like the door lock is held by captive studs on plates inserted through the outer side of wood section 12. The hole for retro fit door handle would pass though this wood section (may already be there) with external handle help in place with wood screws into same section, much like door mirrors are held on with wood screws into door frame elsewhere.
![[Linked Image]](https://i.imgur.com/Tyu7qHL.jpeg)