Well you could note that it is listed as that number by the Maryland Historic Trust in its list of historic sites of the state the same as archeological sites numbers are reported which is like an also known as statement  Please note I never said this factoid had to be mentioned  I said only that it could be mentioned  I certainly agree that if mentioned it is better to explain accurately and informedly what the number means  I agree it is not important to mention it and I would not myself mention it in the text  It could perhaps be included in an infobox like the NRHP reference number is  Editor Pubdog who developed most of the articles on NRHPlisted historic sites in Maryland did not include such numbers in text of other articles and there is no field for it in the NRHP infobox