The majority of those early Model of 1928s were pushed straight into service as soon as they arrived in the UK, shipped to all theatres of the war that British and Commonwealth forces fought in. So we don't come across many pristine Model of 1928's this side of the Atlantic, but this one has suffered from bad storage, as well as war use during its life time.
Reminds me of many recovered Irish Republican Thompsons, and other weapons, which spent more time under the ground than above it.
A good read on the Thompsons supplied and used by the British, and an example of an Irish Republican 3-digit serial number Model of 1921 is shown, in Tom Davis's Great Britain - The Tommy Gun Story. A book I'd recommend and I often refer to, as a good reference on the subject.