plan transfers mid-yr: Short or regular plan year?

Plan transferred its assets into a MEP by 6/30 of a calendar yr plan and is being merged into that plan.

For that reason, the plan as it was is filing a Final 5500.

Just wondering if the final 5500 would file as of 6/30 like a plan term that liquidates its assets, or 12/31 since it's not exactly a plan term.

Would you pick the date like you might for a termination?  One date in the resolution sets the termination date of the plan, but there's an actual final day the assets leave.  I know in a terming plan, that last asset day would be the date I'd put on the 5500 - leading me to suspect something similar in your case where the asset transfer completes the swallowing of the prior plan into the MEP even if that's after the date in the transfer paperwork.