it is extremely difficult to trace this back from today. As you probably had already blocked stock in this year which is as well already transferred in to unrestricted use stock.
So you do not know from the movements whether it is was a transfer to unrestricted from a stock that was received before or after 31.12.
if your stock is batch managed, then it is a little easier to calculate , assuming you would not receive stock to the same batch.
Better you plan already for the next 31.12. and schedule the execution of MB52 transaction so that you can get the info from a printed list. Or you write a program that copies the stock table content to Z tables at the year end if you need those figures later for any report.