I thought it was iffy when it said on the can use within a month. I think they are in brine, will that do in the jar? As it happens herself brought home 3 big jars of Sarson's malt pickling vinegar today. While I was out I'd bought a big jar to pickle some eggs in. The bought ones don't have enough bite for me and seam greatly overpriced.
cheers!
Tom
If they're in brine (and it sounds like they probably are) they won't keep as well after opening, but they should still keep the month anyway.
If they are indeed in vinegar though, not only will they keep better, but you'll have the spicy vinegar to use as a condiment. It goes great on greens (collard greens, mustard greens, etc.)