Type 1, 2, and 3 metric FAL receivers differ in the machining details. IIRC, the type 1 and 2 are similar except for how some of the lightening cuts are made. The type 3 was essentially a cost cutting/strengthening measure and eliminated most of the cuts to save machine time, it is also a bit heavier duty. Not much difference in function between the 3 in a semi as it never receives the pounding a FA milspec FAL does. All Metric FAL kits should fit any of the 3 types of receiver. However, many desire a "correct" rebuild of their kit, so the G1 would take a type 1 to match what it was originally built with. IIRC, Coonan makes a G1 specific receiver with the proper markings and such, you can also get a custom serial number to match the one on your kit. If you just want an economical receiver that works, the Type 3 will work fine with the G1 kit.
The L1A1 kits will need an Inch spec receiver to "drop in". You can modify a few things to get them to work on metric receivers though. The bbl threads are the same on both inch and metric.
Of course, the L1A1 mags only fit an L1A1 receiver due to the large tab on the front, metric mags will work in either one but are a bit floppy loose in an L1A1 receiver.
The unique one is the Israeli FAL receiver, it has it's own spec bbl threads.