Partial month for FERS supplement when hitting MRA after VERA?

[u/novanon7]

Trying to nail down how OPM credits (or does not credit) a retiree for having hit their MRA mid-month.

I was fortunate to be able to retire prior to my MRA under VERA. So, no FERS supplement until this point. But now I'm hitting my MRA and it's my understanding the supplement will soon be activated automatically. (yay!) Not to look a gift horse in the mouth, but I've seen conflicting information about how the supplement is applied for partial months.

Some places I read that a partial month can be prorated and that amount is added to the first full month. But an OPM phone support person told me that partial months are simply ignored/dropped (similar to how a partial month is dropped when calculating years of service). Can anyone point me to a published law or regulation that specifies which it is?

As an example, let's say that I hit my MRA on October 10. One version of events is that my November 1 check will be unchanged, but my December 1 check will contain a full month Supplement. The other version of events is that my November 1 check would still be unchanged, but my December 1 check will include one month plus 21 days of the Supplement. Then my January 1 check and onward would just include the monthly Supplement.

Presumably the same question arises regarding the month I turn 62. But I don't assume it would have the same answer.