paid out terminated plan, then old accounts found

[Guest APSTLC]

Recently a 401(k) plan terminated and all of the assets were liquidated. Recently two pooled brokerage accounts were discovered total balance approx. $11,000. The accounts are titled Money Purchase plan. The Money Purchase plan was merged into the 401(k) years ago. The last year we can find the Money Purchase plan having assets is 1999. It is undetermined what year these assets stopped being accounted for in the annual valuation process. Unsure if it was when the Money Purchase plan existed or after the plans were merged.

Our thought is that these accounts should be allocated as earnings pro rata according to account balance. The question is do we allocate to the people who had an account in the Money Purchase plan in 1999 or do we allocate based upon the 401(k) plan participants balance on the day the plan terminated.

Any regs or guidance you can point me to would be great.