This is an example of an early style 1750's Bess bayonet found with a Crown '5' inspection mark,  it was found in America on a First Pattern Brown Bess musket (46" barrel).  The third step of the mortise is very narrow matching the sight profile of first Pattern Bess muskets,  later muskets had a much wider sight (nearly double the width).  Most Bess bayonets found today whether of an early or later pattern than this example will be found with a wider third step of the mortise.  This is probably due to the longevity of the Brown Bess bayonet design,  which was such that generally even the early Bess bayonets from 1720's could still fit the very last of the Bess muskets made in the 1830-40's.  This meant as newer models of musket were brought into service existing bayonets would be modified to prolong their service. 

The socket is marked with the fraction '7' over '67',  this indicates company '7' weapon '67'.

The key feature of this period is the phasing out of the shield attachment.  Many of the existing bayonets in service were recalled and the Shield attachment was filed off. 

Inside the front of the socket at the junction of shank and socket is a stamped 'B',  this must have been applied during manufacture and has been found on numerous examples of this period.  Its exact meaning is unknown,  but it is assumed to be an inspection mark for the quality of the joint between shank and socket.  The mark does not appear to have lasted for more than 10 or 20 years,  so it may be related to extra inspection work undertaken during the phasing out of shield guard form of attachment.

A rather worn example of the early Crown '5' mark. 

Crown '5' inspection stamps on these early sockets denotes manufacture at the Board of Ordnance manufactories at Lewisham.