I've made a test with @ and it seems pretty useless...
In FS2 retail ships.tbl you have:
...
$Name: @GTDR Amazon
...
$POF file: Drone01.pof
...
$Closeup_pos: 0.0, -2.0, -49
$Closeup_zoom: 0.5
...
$Name: GTDR Amazon Advanced
...
$POF file: Drone02.pof
...
$Closeup_pos: 0.0, 19.0, -190
$Closeup_zoom: 0.5
...
Over that table I've used the next -shp.tbm which switches @ between both ships:
#Ship Classes
$Name: GTDR Amazon
+nocreate
$POF file: Drone02.pof
$Closeup_pos: 0.0, 19.0, -190
$Closeup_zoom: 0.5
$Name: @GTDR Amazon Advanced
+nocreate
$POF file: Drone01.pof
$Closeup_pos: 0.0, -2.0, -49
$Closeup_zoom: 0.5
#End
And voilà, you end with an Amazon looking like an Amazon Advanced, and vice versa.
So it really seems that @ is fully useless.
Could someone confirm it? Or does it have another kind of hidden effect?