Proper EIN Number for Form 5500 for DFE Master Trust

[PenQ]

My Company has a Master Trust that the IRS would call a Direct Filing Entity (DFE).  The Master Trust has the plan assets for several individual Defined Benefit Pension plans that we also file separate Form 5500s.  We also file a separate Form 5500 for the Master Trust itself as required by the IRS.  We use the Plan sponsor's (my employer) EIN for each of the Pension Plan Form 5500 filings, but we are not certain as to the proper way to report the EIN in box 2b for the Master Trust Form 5500.  The IRS has assigned us a separate Master Trust number that they would refer to as the TIN or maybe even the Trust EIN.  The Form 5500 instructions are not very clear with respect to whether the "Employer Identification Number (EIN) in box 2b should be either a)  The EIN number of the Plan sponsor for all the individual Pension Plans OR b) the Trust ID Number that the IRS assigned for this Master Trust.  Does anyone have any thoughts on this?  If so, if someone can actually point to an IRS source, other than the Form 5500 instructions, that clarify this?