I would propose what is enforceable.

That is pretty simple. The JCBA has an effective date. That date is when the new seniority policy took effect. Prior to that date it was not in effect.

There is simply no such thing as a retroactive seniority policy. The guys on the LUS side were not governed by the JCBA. The JCBA didn't even get voted on for well over a year for some of us.