In the playable leak, two SMGs can be found: the SMG1 and the SMG2. The SMG1 was to be the MP5K, but had been replaced by a slow-firing version of the final SMG by the time the early Half-Life 2 was leaked. The SMG2 is an MP7 as well, but it has a 60 round magazine and is capable of one or three-round bursts in addition to automatic fire. It is weak and has poor accuracy but makes up for it with a large amount of ammo per magazine.
It appears that this SMG turned into the SMG featured in the final version, as it has a fast rate of fire, but does low damage, just like the one in retail. It only lacks the built-in grenade launcher.
Interestingly, an unmodified playable leak has the SMG2 use the final SMG model. The SMG2's weapon file must be edited in order for it to use its proper model.
Of note is that the final SMG's world model textures are called "w_smg2.vtf". That means the SMG's model file might have been for the SMG2 at one time, which explains why the SMG2 uses the SMG1 viewmodel in an unmodified playable leak install.
A folder for the SMG2 can be found in the Steam file "source 2007 shared materials.gcf". It contains the SMG2's magazine texture.
In the Half-Life 2 files, several texture files for HUD weapon icons can be found, most of which still containing the icons for almost all of the cut weapons. The SMG2 is among them.