Hmm..an interesting conundrum! Tinlid is the resident expert and I'm sure he'll be able to help you figure it out. For my pennyworth one thing I noticed is that it appears that the storage marks have been overpainted, so at some point it was stored and then painted. They are the rough circular marks at an angle across the top of the shell.
Whether it was returned to stores after Police use and overpainted by the army or picked up by a re-enactor who wanted an army helmet so painted it green I can't say, but I too would be interested to find out.
A nice early Mk2, a lot of the first helmets went to the Police, fire service & ARP. Hard to tell why this one was refurbished.
Since im not collecting Britain and allies, this helm is for sale. Shoot me an PM if interested.