Quote Originally Posted by wiltw View Post
The Ei backs use two latches to hold the back closed, which supposedly is more secure than a single latch.
The Ei backs have an interlock which prevents the dark slide from being removed if the back is not mounted on a body.

The reason for the 135N back is that more emulsion types are available in 135 than in 120/220.
Actually, the two latch type came in 1982, while the locking dark slide came with the Ei backs for the ETR-Si, in 1989.

This is a good source of info. For ETR series history, click on ETR Guide.