A Land Pattern musket bayonet dating to the period 1720-1745 and identifiable by virtue of the decorative shield shaped collar to the socket where if meets the elbow or tang of the blade. This is an extremely fine example and bears an engraved rack number to the socket, although the rack number is difficult to read it looks like N0 176 but I'll let the viewer decide. 
Although the tip is slightly bent the rest of the bayonet is in a remarkable  condition. 
There is a makers mark across the forte which could be researched,
Dimensions (mm)
OL
BL
Socket
MRD
SM
BW
550.0
430.0
102.0
23.0
53.0
31