This is an English trade weight that looks to be in excellent condition. Here are the verification marks from top clockwise:-
Shield - The Arms of the City of London
Crowned 'W' - mark for William IV
Laverpot or ewer - mark of the Founders Company
'A' - mark for Avoirdupois
The earliest the weight can be is 1826 when this type of shield was first used (earlier weights just have the dagger which appears in the top left quadrant of the shield). The crowned 'W' tells us that it was verified and stamped during the reign of William IV (1830-1837) so it is at least that old (but of course it could have been in use quite a long time and lost/discarded at a later date).
From 1826 the ewer mark is often contained in a frame with 18 to left and 26 to right but this weight seems to have retained the older mark. The 'A' is for AVOIRDUPOIS the system used to weigh all goods other than precious metals, stones and medicines.
I'm not entirely certain what size a half dollar is but if you weigh this and convert it to ounces you should get an idea of what this weight was intended to weigh.
