When does the 18-month "seat lock" apply?

For example, if a 727 SO upgrades to 727 FO is he "seat locked" for 18-months and thus prevented from bidding 757 FO until 18 months after being activated...

...OR...

...does the "seat lock" only apply for someone who is currently a 727 FO, who then trains and is activated as a 757 FO, thus preventing them from bidding widebody FO until 18 months after being activated?

I'm hearing two different versions from guys on the line, and also hearing the company is currently taking the first interpretation presented above.