Hi Mate, I've owned Steyrs for almost 20 years and have serviced in excess of 700.
The changes are difficult to define by serial number or year of purchase as quite a few variants were sold at the same time. The main versions are as below (there are other versions like the 10m)
LG100 - Came with a FT style stock with the originals having a barley twist barrel and 'bumble bee' laminate woodwork
LG110 Field Target - FT stock with variations in woodwork design
LG110 HP - black non adjustable stock
LG110 Hunter - walnut stock with adjustable butt pad and cheek piece.
LG110 HFT - FT style stock and all black
LG110 Challenge - Ft style stock, most are all black with the chassis having multiple points where the barrel runs through.
Most changes over the past 2 decades have been cosmetic with the internals staying surprisingly common.
Early 100's had a chamfered firing valve and a recess where the valve seats on the block. The hammer had a flat face/stubby hammer spring and an internal power adjuster which meant you needed to remove the stock to adjust, the hammer spring sat over the top of the hammer 'rat tail' which could result in a 'ping' upon firing.. Later came a rod type hammer with a different hammer spring to fit the external power adjuster, this spring sat inside the hammer so that the ping was eliminated, around 8 years ago (i think) a newer slightly lighter hammer was introduced. There have been 3 type of firing valve with the first being the chamfered one , the next has a light green o ring mounted to make a seal with the newest having a dark green, slightly thinner o ring. The block moved from the recess version after a run of a few hundred to a flat face for the o ring firing valve to seat. The firing valve springs haven't changed for a very long time. The regs are virtually unchanged apart from tiny changes to the piston and to the orientation of the Belleville washers. Later models had a bearing in the trigger pivot with quite a few different styles of FT stock, especially with the after market additions. The barrels have been the main stumbling block IMHO with the early barley twist version being non pellet fussy and an easy cleaning regime, i have one that's 17 years old and still going strong. 2 lengths have been fitted (450mm and 550mm) with the problems being with the 550mm barrels although not exclusively to those, that's why i always advise to shoot a Steyr through a full fill on a zero range before buying second hand. Ah, nearly forgot the bearing for the cocking changed from solid to a version with rollers. Transfer ports have come in Silver/Gold /Green/Black and maybe a few others i've not seen.
Most Steyrs will shoot spot on straight out of the box with no after market gunsmith needed but as they are easy to work on many have been tinkered with and not by people that should be doing it.
If buying second hand try to shoot at least 20 shots through a chrono, 10-15 fps spread is fine as unweighed pellets can actually give a spread of approx 12fps in a tin. Check the grouping for at leat 60 shots, refill and shoot some more. The cocking lever should not grab as excess wear is exhibited by a grab when cocking the rifle, this occurs approx half way through the cocking cycle. Black transfer port is preferred and the external power adjuster.
If you have any more questions, please just ask away mate.