As Dave said you would need some absolute proof other than word of mouth for such things. Relic SD2 stuff is not scarce, there is plenty of it around, a lot originated from Finland initially and now a lot appears to be surfacing from Poland, Latvia and Russia. Of course SD2's were used in large numbers in the Russian campaign and it is not suprising to have these things turn up...........however Grimsby is another thing altogether. Estimated number dropped around 3,000 on one raid. Most dropped on the towns of Grimsby and Cleethorpes, though some did end up in the surrounding countryside of North Coates. However a thorough search ensued and a high percentage of these were dealt with by Bomb disposal and others were recorded as exploded. Numbers were estimated from the number of containers found and so the figures are likely to be fairly accurate.
It is true however that during the early 60's house renovation often turned the odd butterfly bomb up which had been missed, but these were few and far between. My guess is that someone, who perhaps knows little about these things has heard of the Grimsby raid and puts two and two together and comes up with five and a 'good story' to appeal to the collector.
Buy the item not the story is my general motto- if something has provenance then all well and good, but it has to be solid to make it worthwhile.
Having said that, I always ask if I buy something, usually after the purchase to see if any further details can be obtained and sometimes its quite rewarding. There is then no gain to be made by the seller and no need for them to elaborate - the deal is already done. Much of the information supplied could never be proved, but its always worth recording to keep with the item.
I have two bits that I can say hand on heart came from the Grimsby raid (with said provenance) and another two items that are 'reputed' to have come from the raid, being locally obtained I suspect in these cases it may be true, but I have no proof beyond word.
So if something does turn up reputedly from Grimsby and is being sold as such, do a little more research and trust your instinct.
regards Kev