I'd not leave that number plate there, Steve. Someone'll nick it! And probably the chimneys too, if they have pockets big enough.
Actually, in to the late '50s or even early '60s it wasn't unusual to find number plates simply laying around the various loco works. Someone in the railway executive obviously had a moment of blinding realisation when they recognised that such things were being "disappeared". Quite how one smuggled a nameplate out of works I don't know but it was happening and out of depots too, hence the removal of plates before they found a new nefarious home.
Having spent many hours in sheds as steam wound down I can quite see how it would be possible unbolt and "reallocate" a number plate, but I still wonder how those name plates were smuggled away. Of course, I'd never have dreamt of doing anything like that.
Brian