The T series guns (the serial number begins with a T) were made prior to 1970.
They are considered to have superior finishes on them.
Following 1970 the guns were serial numbered with the year followed by a C followed by a number. i.e. 71C12345. In this example meaning that 12,345 had been produced in 1971.
Adjustable sights were offered with these guns.
Around 1977 Browning changed the serial number to 245 followed by the production year in an alpha representation. 245RR12345 245 meaning commercial production 215 was military (I believe) and RR meaning 77 or 1977. See the Browning web page for a breakdown.
Around this same time Browning put in heavier main(hammer) springs 32 lbs versus the former 26 lb springs to better handle hot European ammo.
Around 1983 the Mark II was introduced later followed by the Mark III.
Someone else will have to help me with the exact diffs and dates. Final assembly was done in Portugal. The frames changed from forged to a stronger cast frame, 40 cal was available, available ambi safties, different ramped barrel to better handle hollow points, dot sights and the Mark III added a firing pin block.