A fishy number on a post

Yesterday I posted a close-up of something which looked like a marine creature.

Here is its partner:

Davidss sussed that it was a bent nail, hammered such that the head had folded over.

The two nails fasten a small metal plate, bearing the number 1199, to a post. The post is near Bridge 63 on the Grand Union between Castlethorpe and Yardley Gobion.

My question now is ... why?