Any ideas?

[Upatdawn]

wilfandrowlf said:

I think it's a metal plaque with the number forty three on it.
HTH

what makes you think that?

[Upatdawn]

TheEnd said:

Yep, same fixing as on a water stopcock, which is what it is likely to be related to somehow.

a stopcock mounted on a remote bridge above a canal?

any water pipes would have been along the towpath surely?