Yet another Heiman Smoke Alarm question (Cry for help!)

Hi, I have recently purchaced a few Heiman HS1SA Smoke alarms from ebay, I have spent a day trying to find out how to get them into pairing mode. After a lot of googling it seems you put a pin through a small hole . . . there doesnt feel like anything in my hole.
Could anyone who has used these (or smoke alarms like them) please be kind enough to talk me through the setup process before i throw them out the window.
Any advice greatly received.

This is what I’ve got:

Detailed information, including manual from manufacturer with diagrams can be found here: