My copy of UPS courier has a file in it's "root" called .LSOverride, a text-type file with the content
It's the only one I can see with this file in your list that I have on my machine, and given the content I would wonder if that's what's doing the icon. The only other OXP I can see on my machine with a similar set-up is Military Fiasco, also by Eric Walsh.
When I look for the info of an oxp, I normally see that the default application is Oolite 1.55. (That is still on my computer). On double clicking all those oxp's open this old Oolite version. On a mac I can set an other default application. When I do it for an oxp I can set it to Oolite 1.65. All newer versions are for some reason greyed out in that dialogue window. I once have set those programs to Oolite 1.65. I never knew this information was stored inside the OXP itself. In the old days that sort of information was stored in one of the two desktop files.
I will reset this to the standard program for my next release as 1.65 is still to old. (In the old days until OS7 I knew very well how all things internally worked. But with the Unix based OSX everything has changed)
The .LSOverride file has nothing to do with the icon I assume. But there must be am other difference with a normal folder, even on window machines as on the mac the oxp's created on windows are shown different as normal folders.
I can just assume that some window users create those special folders to be used as normal folders. On the mac they aren't normal folders and with tiger it is decided to also show those folders as plain folders as default. Only when there is mac stuff inside, they are shown as oxp.files.
Probably there is even a preference in Tiger to switch this back to the old way of displaying?
Looking with some old OS7 tools inside an oxp package I noticed they don't contain special hidden files. even the "override" file is not there. It is stored in the desctop file, only the zipper has to put the relevant part of the desktop into the zipfile so an other Mac can use this info.
It is inside the "info.plist" file that is inside the mac version of Oolite itself that change the folders into a package. Somewhere inside the info.plist of Oolite I find:
<string>Oolite Saved Game</string>
<string>oxp</string> <------- this
<string>Oolite Expansion Pack</string>
<key>LSTypeIsPackage</key> <-------- this
This means that a file ending on ".oolite-save" or ".oxp" gets a special handling by its extension alone and not by any hidden file inside. However, this does not answer the situation for it not working on a Tiger system.