Some features of later versions are used but are either restricted to OS4 only, or runtime version checks select the appropriate routines.
Where the lowest required version cannot be determined, the version provided in the ClassAct 3.3 archive is requested.
Bugs may mean these older classes do not work as expected.
In addition, restrict the opening of listbrowser.gadget and the custom Stringview class to OS4 only.
This will happen when attempting to open old 68k libraries with no separate interface file, but may also occur when the interface name or version doesn't exist (although the library version number _should_ prevent that)
