Fidelity Bonds, dates, retroactive & 5500

[TPApril]

post below related to calendar plan year example:

I have understood that although there seems to be no specific mention of effective date of fidelity bond coverage in the Form 5500 instructions, that we use the fidelity bond in effect as of 1/1. In practice, data requests happen after the plan year ends and coverage reported by plan sponsor tends to be as of 12/31.

Ability to purchase retroactive coverage is being advertised. When can retroactive be used?

Makes me wonder if current coverage, increased after 12/31 but prior to 5500 filing date can be used on prior year's 5500?

What date specifically is being looked at for the level of coverage on the 5500 - the 1/1 or the 12/31 level of assets?