I appreciate the question -- and surprisingly enough, your question has a very simple answer.
Within the Termination clause at FAR 52.249-6 -- Paragraphs (c)(6) and Paragraphs (d) MAY BE read as two separate requirements.
Under (c)(6) the Contracting Officer (CO) may already know that there is another Government requirement that can and may use certain items from within the Termination Inventory -- even BEFORE a Termination Inventory Schedule, the SF 1428, is submitted. This property may be Government Furnished Property (GFP) accountable under the terminated contract -- and as such the CO might have the original listing of the GFP that was provided.
While under (d) -- well, that would be all of the OTHER property -- technically TERMINATION INVENTORY, which includes GFP as well as Contractor acquired Property as defined in FAR 45.101 or the GP clause at FAR 52.245-1. And therefore, the clause allows you the 120 days to prepare and submit the SF 1428s.
Bottom line -- you must comply with the directions of the Contracting Officer regarding the Govenrment property. Second, for the OTHER property, NOT listed by the CO, you have 120 days to prepare and submit the Termination Inventory Schedules.