If it's the same as the thing I'm thinking of, they were blocks used to absorb the impact of the vibration of the engines in the testing shed. After the testing, the blocks were chucked into a shed following the testing, and in line with British industrial thinking at the time ('don't throw it away, we might need it...') where they mouldered away until some one found them. The Belfast Convention people gave me one last year - it's affectionately known as 'The Sacred Smelly Brick of Belfast'.
I believe the ones for sale on eBay are nicely polished and have a little brass plaque to say what they are, and may even come with certificate. It's worth bearing in mind though, that it was common practice for factories and shipyards to use these sort of blocks on the floors - they absorbed oil as well as the vibration.