Much as I hope you are right on your theory with these stones, I can't help thinking that the woodcut shows a stone outside the horseshoe, possibly bluestone 36 which is still lying down, The pointed end of the stone would hint that it had fallen inwards, making it closer to the trilithon. Of course it may have been dragged out of the central area to be chopped up but my head says bluestone and a souvenir hunter rather than a quarryman. Of course, as you say, the woodcut and the stones are likely not connected, so there is every chance these stones did come from within the horseshoe. I think deep down the woodcut is a red herring :(

Roman road stones were often worn down over time leaving grooves - could there not be a grooved section on the pointed half beneath the ground that would account for the two bridges - one of each wheel, with natural wear and tear accounting for the groove?