This is a typical India Pattern Brown Bess bayonet,  however the first step of the mortise has been extended by 5mm. 

This picture illustrates the extension of the first step,  the exact reason for this modification is unknown.  One theory is that it was performed to facilitate the easier storage of bayonets on muskets when in racks. By making the extra cut in the first step the bayonet may be located in line with the barrel of the musket rather than to the left of the barrel,  thus enabling more muskets/bayonets to be stored in a given rack space.  It does appear to be quite a rare modification,  although at least one very early shield bess bayonet ^[1] (pre 1750) bayonet is known with this modification .

Blade marked with maker 'I.SALTER',  this is John Salter ^[2],  35 Strand, London, noted between 1800-23.  The blade is also marked with a 'Crown' over '6' inspection mark and a small 'sword' mark.

Crown '6' inspection mark and unusual Sword cutlers mark (which is quite unusual for this late period).

(1) British & Commonwealth bayonets by Ian D Skennerton and Robert Richardson,  B43 page 31, see also page57,  ISBN 0 949749 04 4

(2) The Brown Bess Bayonet 1720-1860 by Graham Priest, page 150 , ISBN 0-946696-05-5