The stamps belong to your then employer. It might not be important if they are of little value, but what if one of them hapens to be worth a great deal of money, you sell it and the fact is widely published, being brought to the attention of whoever employed you? What might their reaction be?
The same applies to what you folks over there call garbage - and the stamps probably fall into that category. You cannot remove from a container something that has been discarded by someone and claim ownership. I know it happens daily around the world, but there have been a few prosecutions. The people who empty or take away the containers sell whatever they can. It is part of their business to do so.
Harry, I once worked in a legal office where a young staff member had acquired a heap of old unused stamps from his grandfather. They were worth no more than face value in the stamp market, and it was decided that he could sell the stamps to the firm so they could be used. They were applied at the then current postage rates. It was never challenged by the Post Office. They would have made interesting collector's items (as yours will) if the present day postmark is shown on a 40 y.o. stamp.
I understand that in some places you can buy stamps for the present letter rate amount and use them at any time in the future even if the postage charges increase.
