From my experience - and MBJ would probably tell you the same thing - what you see in those frames in the NTSC version are the burn marks "cleaned" up by the '93 THX process. If I recall correctly, you can see the same type of thing between the Japanese Special Collection and the Definitive Edition LDs. The JSC will have a scratch or a spot or something in one frame, and in the DE it's still kind of there but mostly not. Things like those burn marks, however, are not on the JSC.

Based on that, I'd say that the PAL and NTSC THX LDs are from the same transfer. I would also guess that the pre-THX LDs were possibly transfered from the same print, but from an earlier transfer.