1800'2 darby?

i found what looks like the picture of fitzwilliam & son darby on this web site. it has a key that screws in? they looks really old and i've never seen one like it but i don't know much on the subject. can someone tell me what they might be and what they might be worth?

Do they say FW.SM on them? otherwise they could have been made by almost anyone.

If you can upload a Picture or email me one it would help.
Paul