A plan has its first plan year-end on 8/31/04. Therefore, the minimum funding is due 8 1/2 months after that date.
Their corp tax return is due 5/16/05 since 5/15 is a Sunday.
They make their contribution on 5/16/05.
Is this a funding deficiency requiring a 10% excise tax?
My recollection is yes, but perhaps there is an exception someone knows of.

How does one prepare the Sch B in this case?

A contribution deadline is not extended when it falls on a non-business day.

IRC 7503 provides an extension to the next business day for *filings*, but the extension doesn't apply to qualified plan contributions.

In the case at hand, the contribution should not be shown on the Schedule B, resulting in a deficiency.

If the contribution were being mailed under the above scenario, would the postmark need to show 5/14/05 or would 05/16/05 be sufficient because the postal service is closed for business on 05/15?

You'd need a 5/14 postmark here to evidence that the contribution was made on time.

This may be the correct result, but don't go so fast that you overlook other facts. Don't have to use the date received by the trustee, if the payment was mailed (or wired?) on an earlier date.