Ok, I'll give 'er a go.
If I'm not mistaken, the third gen has "Millenium" on left side and "PT111 Pro" on the right.
Second gen has "Millenium Pro" on left side and "PT111" on the right. Also if it is a 3rd gen, it will be SA/DA whereas there is takeup on the trigger pull before firing, then reverts to DA(assuming you are dry-firing). 2nd gen will be no takeup on the trigger and will always be in DA(DAO).