Noncovered Securities with Undetermined Holding Period

I have (5) items on my 1099-B from Morgan Stanley which classify the transactions as Non-Covered Securities with Underdetermined Holding Period.  I can determine the Date Acquired for the shares in question (iShares Silver Shares ETF), but not sure what Cost Basis that I should be using (since the shares sold were likely partial shares with proceeds used to cover investment expenses from the sale of bullion shares  (see note below). Can I just use the price per share of the original Cost Basis as factor to come of with the Cost Basis for these partial shares?

 

NOTES FROM 1099-B:

 

# Morgan Stanley reports transactions in the Noncovered Securities with Undetermined Holding Period Section of the 1099-B portion of the 1099 Consolidated Tax Statement
when the holding period or cost basis is not readily available, and therefore Morgan Stanley is not able to determine the appropriate holding period designation for these
transactions. Please consult with your tax advisor.

 

CT - The proceeds amount is the sales price of the bullion sold by the Trust to pay investment expenses. Proceeds from the sale of bullion are reported on Form 1099-B.

 

Thanks in advance.