a puzzle

[NeilUK]

I recently was bequeathed by a friend, now sadly dead, a most unusual socket sword bayonet. Overall its length is 81.5cm, with a blade of 69cm, and that despite the loss of its point. The blade is back edged with a narrow fuller for 3/4 of its length just in from the back. The blade has a single mark - a crown over the number '2', a standard British government acceptance mark. The socket is marked '11' and '81' which suggests Company 11, rack number 81. I have been able to discover that it is probably a Pioneer sword converted into a bayonet, hence the extreme length. What do the edged weapon experts think?
I have no information as to its provenance, my friend had never mentioned it and his family know nothing about it.
Neil