14'' Shaft 1 peice was the first eclipse kinda stock barrell fitted to eclipse cockers and the Ego5, 6.
They also made 2 peice Shaft 1 barrel which was used on the Nexus, XSV EGOs, including the SL66 and the higher end cockers eg Nexus / DC2
Shaft 2 improved on Shaft 1 with a more robust screw threads between the front and the back. This was introduced on the EGO 7 marker.
Shaft 3 is a different milling / porting than the Shaft 2 other than that they are interchangeable
Shaft 4 is the newest model with a longer control bore (back section) which is not interchangeable with any previous Eclipse barrel system. this has debuted on the GEO2.
Shaft barrels are all typically the stock barrel system on EGO and GEO markers. Most models have been avaiable to buy seperately or as kits but are usually limited / produced in small numbers for short periods. Therefore you may struggle to find Shaft 2 peices as NOS (new old stock). Notable exception so far is the Shaft 4, im sure at some point backs or maybe whole barrels will sell, but for the time Eclipse are packing all stocks with there GEO2 markers.
Note, that SL EGOs (from the SL-74 onwards) come with SL variants of either the 2 or 3 (more aggressively milled, lighter) but are interchangeable with the standard shaft 2 and 3 barrels.
Hope this helps. If someone spots and inaccuracy please let me know. This isnt meant to be completely comprehensive.