Little update, my friend told me his grandad used to 'make precision instruments' - might mean they're some kind of bench block?
He's going to dig up some more detailed info for me, but hopefully that'll give one of you enough of a clue!
Thanks for all your replies! I'm not sure what he used to do, but I don't think he had any particular trade - will ask my friend when I get a chance.
In a thread on another forum there's been the following suggestion, which a few people have agreed with:
"I think that these are...
I was given these three metal blocks by a friend earlier, they were found in his recently deceased grandfathers house. All three are near enough identical in size, with two featuring similar markings on the top (the wrapped one shares a similar pattern with one on the left) while the third has a...