In our house (believed to be late 18th century) a number of the original door lintels were marked with cast iron markers made up of a letter and number. Each digit is on the head of a spike driven into the wood. The one pictured (L7) was on a wooden lintel on a downstairs doorway leading to an attached outhouse which was at one time a smithy. The lintel was removed during some renovation work (under building regs scrutiny otherwise it would have remained in place) and has been made into a garden bench. I had a fancy that it related to some kind of taxation or rateable value system similar to window tax and based on the number of rooms in the property by which L7 may refer to lower floor room 7. Can anyone shed any light on these marks?