Raine Borg has an amazing web site about locks and keys through history. And it so happens that he’s made reconstruction drawings of how keys identical to the one me and Per Vikstrand found in Torstuna recently were used. It’s not a padlock key: it’s most likely for a lock mounted permanently inside the lid of a chest or a door.
Lena Thunmark-Nylén’s Die Wikingerzeit Gotlands informs me that the key type dates from the 11th and 12th centuries. Thus, alas, a bit too late to tell us much about pagan activities in the Field of Thor.
Thanks to Raine for permission to reproduce his drawings and to Tobias B. for tipping me off.