Obviously used for some kind of Band PA or similar as heres another pair in Chi-town.So it would seem that it isn't that uncommon as both have the same drivers/horns too.

Altec 1505 Horns 291-16B Drivers 30166 Throats 421-8H - eBay (item 190544274709 end time Jun-22-11 19:00:30 PDT)