My 2 cnts
When you hold the stripped frame upside down,
there will be a small pin sticking out of the inner side acrosed from the ejection port, almost all the way at the back of the frame, just forward of the part of the frame the butstock touches.
This pin is what holds the ejector in place when the barrel is off the frame! there should also be a small hole in the frame for the tension spring.
I don't know how old your 1200 is,
there are I don't like to think how many revissions of the ejector, at one time I had ten ejectors, and they where all a little diferrent for the revissions, on early models there was no coil tension spring.
Also when installing the barrel, make sure the ejector end goes into it nook, between the barrel extention and barrel, if it gets into the chamber and the bolt is forced when closing it will ruined!l