You done real good, a
Lithgow
is an excellent rifle and a good first one. I have a 1918 lithgow too, my first enfield. I now have 7, each one a separate bit of history and a fine shooter.
MD stands for military district, as in 1 MD,2 MD, 3 MD etc, each MD was a different state, 1 MD was Queensland, 2 MD was either Victoria or NSW, or in that area. MD rifles were the stockpile held in case of invasion of the homeland. Many post war surplus rifles came from those stocks. A lot of WW1 rifles ended up being re furbished and held ready in case the Japs invaded.
CMF = ?, etc on the right butt stock, its all a common branding on lithgow rifles.
A marking HV means the rifle sights were configured for high velocity ammo, which was a slight improvement on 303 ammo from circa 1900.
SHTle means short lee enfield, or short magazine lee enfield = smle, its a model designation, previous rifles were the long lee. The no1 mk3 was initially looked upon as a carbine concept, ie cutting back the rifle and sacrificing some long range ability for more manuoverability, supposedly.
I don't know why you have a second serial number on the rifle, but its not uncommon on enfields, it may have been reserialed by an importer, or by the armory doing parts replacement. If you see the initials JJ Co Ny Ny in little letters behind the bolt its the importers mark who brought a lot of enfields into the US.
Lithgows are a nicely made, nice shooting rifle, with plenty of history and should give you years of shooting fun, and afterwards it'll be worth more than you paid for it. Doesn't get much better than that.