Yep, a lot of crud from running tap water without rust inhibiting anti-freeze.
Could even be sand from the molding cores that was not fully cleaned out at the foundry.
Not an XK, but here is what I found behind the core plugs in my Mark V head.
These wires were to hold the sand cores together before the iron was poured into the mold at Midland Motor Cylinder Co. They either forgot or didn’t bother to pull them out afterwards.