Can anyone identify this object?

[PatB]

42 minutes ago, Wheatley said:

... so when you rest one end of your spirit level on the red block and the other on the railhead, it should be level. If it isn't then something's moved ! And on that one the running edge should be exactly 1055mm from the plate. I think the "00" means no cant but I'm starting to exceed the limits of my memory here. 

I thought it might mean you need to add a bit to the 1055mm dimension to account for the gauge being a bit narrow.