The first problem is the fact that running Period End or Year End to move transactions to history should not have deleted any pending Ledger transactions waiting to be retrieved, but then Payables 8.1F is over a decade old so maybe I'm not recalling that versions idiosyncrasies correctly.

There definitely isn't any process to move transactions from History to Current, recreating the Ledger transactions in the process.

An Adagio Consultant would be able to help you recover any lost transactions from a backup without losing the current data.