The prerequisite doesn't matter for equiping an item only for using that item. You can equip any item at any time regardless of whether or not you meet the requirements. However, you won't get the full benefit of that item if you don't measure up.
So, yes, you were getting some defense from the shield. But, no there's no way short of some really annoying
math and experimentation to find out exactly what the actual bonus was. It was lower than the AL you'd get with Tactics 7 and, as stated above, something like the base or starter shields you'd find plus any internal bonuses or penalties (I don't think these exist any more but at one point I was sure that just as items might have an invisible +1 there was an invisible -1 or more possible, too).