Judging by my own experience so far, there are at least two “mix sources”, while the newer one might have been derived from the old one by cleaning it up. Without any offical inside information, it’s difficult to say, though.
Former LDs and DVDs seem to share the same mix source with differences in mastering though:
http://www.videophile.info/Graphs/JP/JP_01.htm
The slight distortions on the dialogs, I mentioned several times here and which for reasons unknown, virtually no ones seems to care, are shared by at least the AC3 and DTS LDs and the Cinema DTS of Jurassic Park. Hence, slightly distored dialogs or not, this is most probably how Jurassic Park was simply meant to sound back in 1993.
The 5.1 LDs (whether AC3 or DTS) are nonetheless considered to sound superb despite that there is some debate whether the DTS LD has the correct mixing levels now or not.
On the first Blu-release and its 7.1 mix, the dialogs sound a lot cleaner to me but the mix and the placement of several effects (most easily to recognize during the first raptor scene) is definitely different which makes me wonder why “Disclord” in his review came to the conclusion that they essentially left the mix as it is and only seperated the back channels.
Still more than 23 years later, there seem to be still mysteries unsolved about Jurassic Park.
Regarding Jurassic Park showing up on eBay: patience! Got two copies of it myself at a surprisingly reasonable price (at least compared to highly collectables like “Fight Club”). 😉