Evert has summed up the main differences very well. All I’d add is the later Trackers were fitted with Webley’s spring guide (instead of a rubber washer at the back of the spring) and a few other subtle differences, such as the cocking link fulcrum pin was changed from riveted ends to a top hat pin with a tiny circlip. The machined profile of the back block was also different on the Tracker.

The Ospreys were very slammy and low powered mostly due to those bronze composite piston rings, that wore down and ceased to create an air seal. As has been mentioned a Tracker Piston will fit which opens up options for fitting various parachute seals.

Cheers
Greg