These are not the greatest photos, but will provide a partial answer to your question.
In this photo, a model 300 is on top, and a 300S on the bottom. Note that the 300S lever stops at the front of the loading port. The 300 lever is longer, reaching to the front of the receiver tube. The 150 lever was longer still, reaching the front of the barrel mount block.
In the shot below, the 300 is on the bottom and the 300S on the top. The older model 300 has a hinged "car door" kind of latch, i.e. the tip of the lever swings away from the gun to release the lever catch (the 150 also had this type of latch).
The 300S lever tip is fixed, not hinged. Note the inset bar in the thickest part of the lever, just behind the gun's loading port. One pushes down on the small rounded protrusion on the front of this, to release the lever catch.
NOTE, the transition from model to model was not entirely clear-cut. Some late model 300's were made with the later 300S type lever. I've seen one model 300 which also had the shorter 300S barrel, but retained the older stock and trigger. Other odd transitional combinations of features turn up on FWB's from time to time.
The initial model 150 was similar in appearance to the 300, but the barrel was shorter, the grip was checkered instead of stippled, and again the cocking lever was longer.
FWIW, this model 300 is easily the best-shooting air rifle I own, in spite of its slightly shop-worn appearance. It is effortlessly accurate with any pellet I stuff into it (the 300S is a bit more finicky about what it eats), and shoots light pellets in the 680's in spite of the fact that I can quite literally cock it with my little finger. Just a fantastic old air rifle.
I had just read an article about the FWB match air rifles, and this was mentioned.