Hi Bacco,
yours is 100% right, looks like Doncaster soot on the back. I seem to remember there being a couple of versions of this sign. The one I had, had a packet attatched, sadly I don't have an image. Might be able to get one- If I can I will forward it to you.
I know several of the various signs from this find, had packs removed- probably done at the time.
rusty