Hiya mate, it could be tax exempt! I'll explain why.
I own a 109 series 2a ex military ffr truck and when i bought it it had a 1975 reg plate, but when i checked the log book it said that it was built in 1972 but first registered in 1975 with civilian uk plates!
I checked its military history with the aid of the military VIN plate and its military reg number that was stamped on it, and, obviously, it had spent 3 years in the mod as a standby vehicle and had seen very little service, before being ousted by a newer series 3, I imagine!
Basically, when these vehicles are ousted from the military, they are registered in that year, rather than than the vehicles manufactured year.
Also, my landy has the all metal dash, making it one of the last series 2a's built. A few series 3 landys were tax exempt, they had the plastic dash.
So, if yours is a plastic dash ser 3 variety on 1974 plates chances are its not tax exempt as it would have had a very short military life before being registered with civvy plates.
If you have the original military VIN plate, there are websites that you can get all the vehicles military history, this may help. Dates and suchlike!
Other than that, check your logbook for that 1972 manufactured date. It just may be, that its taxation class has never been changed from PLG to HISTORIC VEHICLE,
Hope this helps,
Landy109