It could well be the ground type. All the rounds I've found have been surface finds on grass land, I've also found frags of 3" mortars too but never a complete one. When I was in Iraq we found every size going, 2", 3" 81mm and some bloody big stuff. One of my last jobs was to a massive dump of French HE rounds, there must have been about 200 in a 10meter square area. All live but with no fuzes or plugs fitted, the locals had been melting the HE out to supposedly start there fires.
I know what you mean m3 but I too am like Danny. I've come across many an 'unused' mortar round and determined that, despite the fact the gas holes are open, they were just dumped by the guys on the range.
After 60 years in the ground the water eats the cardboard tube away and makes it look 'fired' (you were right about the safety cap by the way....my mistake ) and, of course, leaves the fins intact.
So I still think it has a high liklihood of being unfired, especially seeing as all the paint and markings have disappeared. This can also be a result of being left in the ground for 60 odd years.
Last two trips to 'my' range I have found 2 inch smoke rounds within 2 feet of each other. One had virtually no markings left and all the propellent holes were open BUT the safety nose cap was in place. The other looked like it had been dropped a week last thursday with every marking still readable and the colours vibrant. It too had totally open propellent holes. The reason for the difference ? The good one was on the crest of a dune, the bad one at the base of the slope.
Water can be a real sod....