FN-FAL, G1, L1A1: basically all the same, commonly referred to as 'FAL' (pronounced F-A-L), L1A1 was the British version made with inch-based measurements while the G1 and FN-FAL are made using metric based measurements. Whole assemblies will interchange between the rifles, but the sub-parts might not (for example, lower recievers, with fire control groups installed, will swap and function, but triggers will not always fit in different lower recievers). G1 has a flash hider held on by a bayonet-action lug (rotate it about 90 degrees to lock onto a big lug on the barrel, doesnt count as threads so you dont have to use a lathe to build a postban G1, just twist and remove the flash hider), while the FN-FAL had a threaded barrel and flash hider.
G3 and Cetme are basically the same, with the Cetme having been designed in Spain by German engineers in hiding after WWII. When they returned to Germany they worked for HK and refined the design into the G3/HK91.
Kharn