Having searched for more info on the 84th ....I found an image of a uniform button dated 1855-1881 .....the style of the numerals are the same as the bayonet...although the etched '4' on the pommel is slightly more 'rubbed' than the '8', enough is visible to see they are identical
Its always exciting to have a piece of history....no matter how rusty...in your hands
Regards David