Hmm, in theory this is not supposed to happen, because the IRS and DOL definitions of "governmental plan" are supposed to be identical. However, as you point out, the different agencies may interpret the same requirements differently.
The Department of Labor Form 5500 requirement would in theory be governed by ERISA Title I, not the Internal Revenue Code. However, depending on your willingness to take an aggressive approach, you might use the IRS private ruling as a basis for not filing a Form 5500 with either IRS or DOL. Indeed, you might want to check back and see whether the private letter ruling was cleared with DOL; most of the ones which concern common issues are. In any event, it would appear to be difficult for DOL to impose penalties for nonfiling of the Form 5500, since those penalties apply only if there is no "reasonable cause" for the nonfiling, if the IRS had held the plan to be governmental.
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