Recognizing Necessity

Tonight's post is (even) more of a ramble than usual - a question I'm just starting to get interested in and would like to know (and think) more about.

Say there are some a priori contingent truths (e.g. The metre bar is one metre long). If that's so, how do we recognise necessity when we see it? It can't just be that, when we realize we have special a priori access to some truth, we thereby (gain all the information we need in order to) realize that the truth in question in necessary.

Here's a quick answer: suppose that coming to know a proposition a priori is a matter of realizing that a certain kind of relation holds between the concepts involved. Then there doesn't seem to be any obvious reason why it should always be the case that, when the right sort of relation holds between the concepts in p, that sort of relation also holds between the concepts in Necessarily p. We come to know p a priori by recognizing something about one set of concepts, and we come to know a priori that Necessarily p by recognizing something about a different set of concepts.

I like this sort of answer in principle, though it can't be quite the whole story. One thing it doesn't do is explain why there is so often a connection between realizing that you have a priori knowledge that p and realizing that p is necessary. Apart from that, though, I wonder whether people are inclined to think it is lacking in some other way (and/or to have views as to what should we say to address the fact that there is usually a connection between a prioricity and necessity)?