If you put a late 80s/early 90s serial numbered or no serial low end Japanese Greco side by side with a no serial number low end Korean Greco then the differences are self evident.
The Korean pickup routing is different, the shielding paint in the pickup and control cavities are different, the carved top has a different shape, some of the pots have Cor-Tek (Cort) on them, the necks sometimes have multipiece scarfed headstocks especially on the SG's, the general look is different, their headstock design and shape is different and if a no serial Greco has these features than it's from Cort whether it has Cor-Tek pots or not because the Korean Greco LPs and SG's all look the same and have the same features.
The Cor-Tek pots just show where they are from, Cort.
If you want to know where a no serial Greco Firebird is from then you would have to do a similar comparison.
If you put a late 80s/early 90s low end G serial OBG side by side with a K Orville there are really no differences besides the nitro finish and maybe better wood selection for the G serial OBG and some other things like the 3 piece tops on some K Orvilles.
If the K Orvilles had the same sort of differences to the G serial OBG's like the Korean Grecos do in regards to the Japanese Grecos then the K Orvilles would more than likely be Korean but the K Orvilles don't.
The K Orvilles have the same routing as the G serial OBG's, they have the same mixture of tenon length, long, medium long and medium, they have the same non scarfed headstocks, the carved tops look the same, they have the same pots, they have the same headstock designs and shapes.
Just because some K Orvilles have 3 piece tops is not a big difference, it's just a money saver to reuse maple bits that would be thrown away.
The SG's are the best to compare for the G serial OBG's and K Orvilles because they are simpler and anyone who has seen a K Orville SG and G serial OBG side by side would be hard pressed to tell any differences between them besides the nitro finish and pickups.
So the K Orvilles don't have very different features from the G serial OBG's so they are Japanese.
There are just too many similarities between the K Orvilles and G serial OBG's for them to be from different factories.
The G serial number OBG's and the no G serial OBG's show differences in the headstock designs and shapes as well as other things and that's because the G serial OBG's are from Terada and the no G serial OBG's are from FujiGen and the differences are quite apparent.
There are other differences as well like FujiGen didn't make Orvilles with 3 piece tops but did make those low end photo flame Orvilles.
A guitar from a Korean factory in the late 80s/early 90s can't help but be different in some features to a guitar from a Japanese factory in the late 80s/early 90s especially if the guitar is the same brand and same sort of model.
Just comparing them will show the differences.
So if the K Orvilles didn't match the G serial OBG's but had features like the Cort Grecos for instance than they would be Korean.
Just because a guitar is Japanese doesn't mean that it can't be a pile of crap and vice versa just because a guitar is Korean doesn't mean that it can't be a great guitar.
The no serial Grecos that don't have the Cort features listed above have features in common with the Japanese serial numbered Grecos like the same routing, same pots, same carved top shapes etc so these no serial Grecos are not from Cort but from a Japanese maker.
They have too many similarities with the Japanese serial numbered Grecos for them not to be Japanese.