It is indeed a Model 5 variant made by Fedalcode, no real good reason why the second louver was omitted but I have see a number of these including one at my old department (which was removed before I started running there). It sounds like your standard Model 5, here is an example from Tremont, Pa.. it originally had a standard Federal cover on it but it came apart so Warningsirens.org board member replaced it with a cover from another siren in the county that stopped working. This is the same 5T that is heard on YouTube sounding for 3 to 5 minutes and the guy complaining about it.
https://goo.gl/maps/63a5tye3GR52