Here are a couple of verses coming from two different versions of the Gower Wassail:

Now master and mistress if you are within
Pray send out your maid with her lily-white skin
For to open the door without more delay
For our time it is precious and we cannot stay


There's a master and a mistress sitting down by the fire
While we poor wassail boys stand out in the mire
Come you pretty maid with your silver headed pin
Pray open the door and let us come in

This would (somewhat disappointingly!) suggest that the reference was to the master and mistress's maid and that, for some reason, she had a silver-headed pin!! Maybe it meant she was wearing her best: maybe it suggests the prosperity of the master and mistress that their maid should be able to afford a silver headed pin.

Any other ideas?

John